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• Published: 28 June 2024

Magnesium hydroxide addition reduces aqueous carbon dioxide in wastewater discharged to the ocean

• Vassilis Kitidis   ORCID: orcid.org/0000-0003-3949-3802 1 ,
• Stephen. A. Rackley   ORCID: orcid.org/0000-0003-1252-5376 2 ,
• William. J. Burt 2 ,
• Greg. H. Rau 2 ,
• Samuel Fawcett 3 ,
• Matthew. Taylor 1 ,
• Glen Tarran   ORCID: orcid.org/0000-0003-3695-5151 1 ,
• E. Malcolm S. Woodward   ORCID: orcid.org/0000-0002-7187-6689 1 ,
• Carolyn Harris 1 &
• Timothy Fileman 3  

Communications Earth & Environment volume  5 , Article number:  354 ( 2024 ) Cite this article

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• Climate-change mitigation
• Marine chemistry
• Pollution remediation

Ocean alkalinity enhancement (OAE) reduces the concentration of dissolved carbon dioxide (CO 2 ) in seawater, leading to atmospheric carbon dioxide removal (CDR). Here we report laboratory experiments and a field-trial of alkalinity enhancement through addition of magnesium hydroxide to wastewater and its subsequent discharge to the coastal ocean. In wastewater, a 10% increase of average alkalinity (+0.56 mmol/kg) led to a 74% reduction in aqueous CO 2 (−0.41 mmol/kg) and pH increase of 0.4 units to 7.78 (efficiency 0.73 molCO 2 /mol alkalinity). The alkalinization signal was limited to within a few metres of the ocean discharge, evident as 27.2 μatm reduction in CO 2 partial pressure and 0.017 unit pH increase, and was consistent with rapid dilution of the alkali-treated wastewater. While this proof of concept field trial did not achieve CDR due to its small scale, it demonstrated the potential of magnesium hydroxide addition to wastewater as a CDR solution.

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Introduction.

Limiting global warming to 1.5 °C requires a drastic reduction of anthropogenic greenhouse gas emissions and further carbon dioxide removal (CDR) of 1–10 × 10 9 t atmospheric CO 2 year −1 by mid-century 1 , 2 . Ocean alkalinity enhancement (OAE) is one of the proposed solutions for CDR 3 . Dissolution of CO 2 in the ocean already takes up circa 26% of anthropogenic CO 2 from the atmosphere (2.9 ± 0.4 × 10 9  t C year −1 ) 4 , 5 . CO 2 dissolves in and reacts with water to form carbonic acid (H 2 CO 3 ) which further dissociates to bicarbonate (HCO 3 − ) and carbonate ions (CO 3 2− ), balanced by hydrogen cations (H + ). In this context, ocean alkalinity can be thought of as a measure of the capacity of seawater to neutralise carbonic acid to HCO 3 − and CO 3 2− , balanced by cations other than H + . Under steady state, these reactions establish an equilibrium between CO 2 in air and dissolved inorganic carbon in seawater (DIC; the sum of CO 2 , H 2 CO 3 , HCO 3 − , CO 3 2− ). At present, increasing atmospheric CO 2 is continually driving CO 2 influx to the ocean and increasing CO 2 , acidity, and DIC which is largely removed from the surface and entrained in the deep ocean circulation for centuries to millennia. OAE also drives CO 2 into the ocean and increases DIC, but instead reduces acidity. This mimics and accelerates natural mineral weathering and ocean alkalization, thereby increasing the capacity of ocean CO 2 uptake and long-term storage of DIC 6 , 7 , 8 . Addition of alkali to wastewater has recently been proposed as a vehicle for OAE and CDR 9 . Effluent from wastewater treatment is typically rich in biogenic CO 2 , creating a water to air concentration gradient that drives efflux of CO 2 to the atmosphere. In principle, alkalinity enhancement of wastewater re-establishes the carbonate equilibrium such that dissolved CO 2 gas (which can readily exchange with the atmosphere) is converted to dissolved bicarbonate and carbonate ions (which do not exchange with the atmosphere). Where treated wastewater is discharged to the ocean, alkalinity addition can thus result in: (1) ‘avoided emissions’ from the reduction of biogenic CO 2 emissions to the atmosphere and, if sufficient alkalinity has been added, (2) ocean CDR downstream of the wastewater discharge.

Numerical models have shown that OAE can effectively achieve CDR and reverse ocean acidification at the global or basin scale 10 , 11 , 12 , 13 , but their predictions remain untested by field data 14 . Here, we report the results of laboratory experiments and a field trial of alkalinity enhancement in wastewater using a commercial magnesium hydroxide suspension (Mg(OH) 2 ; hereafter ‘MH’). We show that MH addition leads to a reduction in aqueous CO 2 concentration in the final effluent discharged into the coastal ocean.

Wastewater alkalinity enhancement

A set of preliminary lab experiments were conducted with ‘final effluent’ from the Hayle wastewater treatment works (HWTW) in order to constrain the field trial dosing rate. A commercial MH suspension (53% as 2 μm solids; 98.5% of solids as Mg(OH) 2 or 522 g/L Mg(OH) 2 ), derived from calcined magnesite, was added to treated wastewater at different concentrations at 22 °C, preventing further air contact (‘closed system’). Increasing MH fraction in the MH-wastewater mixture led to a reduction in dissolved CO 2 (CO 2 (aq) ) and increase in TA from 5.5 mmol/kg up to 8.4 mmol/kg at 1% v/v MH suspension in wastewater (Fig.  1 and supplementary Table S- 1 ). DIC remained unchanged as expected from conservation of mass (CO 2 was converted to HCO 3 − and CO 3 2− , but the sum remained the same), suggesting that no precipitation of inorganic carbon took place in these experiments. The pH of the wastewater mixtures increased with increasing MH fraction, in agreement with previous studies for the addition of laboratory grade Mg(OH) 2 and the Mg-rich minerals brucite and olivine to seawater 13 , 14 , 15 , 16 . Thereby, 0.01% and 0.1% MH suspensions in wastewater effected pH increases of 0.2 and 1.2 units respectively. A logarithmic curve was fitted to our experimental pH data: pH lab-experiments  = 0.316 × ln(MH % Volume Fraction ) + 9.26. Total suspended solids (TSS), quantified immediately (TSS in ) and 18 h after MH addition (TSS 18h ), demonstrated dissolution of particulate MH. We assume that ΔTSS=TSS in -TSS 18h represented the sum of dissolved Mg(OH) 2 plus that consumed by H 2 CO 3 in wastewater. Given a molecular weight of 58.3 g/mol, we calculated the ΔTSS-equivalent [Mg(OH) 2 ] as 0.5, 3.8, and 14.2 mmol/kg for the 0.01%, 0.1%, and 1% v/v MH additions respectively. Since these estimates exceeded the solubility of Mg(OH) 2 in pure-water (~0.3 mmol/kg), Mg(OH) 2 must be consumed by the abundant H 2 CO 3 in wastewater. The conversion of CO 2 to HCO 3 − through this reaction was corroborated by a geochemical speciation model using the concentration of alkalinity, pH, CO 2 partial pressure (pCO 2 ), temperature, and added Mg as input parameters 17 . Finally, there was no evidence of struvite or vivianite precipitation which could cause operational disruption of the HWTW. Phosphate and ammonia data, which decreased conservatively with increasing MH fraction, would have decreased non-conservatively if precipitation had taken place. Our experiments determined a mixing ratio of ~0.02% v/v MH suspension in wastewater for the field trial (equivalent to 104 mg/L or 1.8 mmol/kg Mg(OH) 2 addition).

Water properties against the fraction of MH suspension in wastewater. TSS was determined immediately (<3 min; light blue in top right panel; TSS in ) and 18 h after the addition of MH (dark blue in top right panel; TSS 18h ). Dotted lines represent best fit for pH and TSS.

Following the lab experiments, MH was pumped directly into ‘final effluent’ at the HWTW on three consecutive days (18th−20th September 2022) for ~8.5 h per day at 0.058 L/s into a wastewater flow of 270 ± 70 L/s (±standard deviation) giving a mean mixing ratio of 0.021 ± 0.008% v/v [110 mg/L or 1.9 mmol/kg Mg(OH) 2 ]. Monitoring stations, upstream and downstream of the addition, were separated by approximately 200 m with a mean residence time of 22 min (MH addition to CO 2 /pH change was 16–33 min). Thereafter, effluent was discharged via an 11.3 km pipeline (residence time of ~9 h) terminating 2.5 km offshore in four diffusers at 23 m depth.

The addition of MH to treated effluent at the HWTW led to a decrease in xCO 2 (the mixing ratio of CO 2 in air-equilibrated-wastewater) and concomitant increase in pH and TA (Fig.  2 ). Prior to and between MH additions, downstream data were consistent with the upstream data with average xCO 2 of 1.36 ± 0.20%v/v (range: 0.6–1.5%v/v or 6000–15,000 ppmv), and pH in the 7.2–7.6 range. During MH addition, xCO 2 decreased consistently with average xCO 2 of 0.36 ± 0.28% and to a minimum of 0.038% (or 380 ppmv × CO 2 ), i.e., close to the atmospheric xCO 2 mole fraction (~410 ppmv). pH increased from 7.38 ± 0.09 (upstream) to 7.78 ± 0.17 during MH addition. There was no significant difference in DIC between paired upstream-downstream samples during MH addition [DIC range 5.4–6.5 mmol/kg; mean difference ( ± st.error) = 29 (±27) μmol/kg], confirming the conservation of mass for DIC between these stations (supplementary Table S- 3 ). In contrast, addition of MH suspension increased TA consistently by up to 943 μmol/kg, equivalent to a 17% increase, compared to concurrent measurements upstream (mean upstream TA: 5591 μmol/kg; mean TA difference between paired upstream/downstream samples was 560 μmol/kg). From the mean change in xCO 2 and CO 2 solubility at 18 °C 18 , we calculated a molar CO 2 change of 410 μmol/kg. This gave an efficiency of 0.73 mol CO 2 reduction per mol TA increase or 1.47 mol CO 2 per mol MH for the average alkalinity increase (560 μmol/kg) in agreement with published efficiencies of 0.6–0.8 mol CO 2 per mol TA 13 , 15 . TSS loads at the upstream (59 ± 25 mg/L) and downstream stations (66 ± 45 mg/L) were statistically indistinguishable ( t test, p  > 0.05), confirming significant dissolution of the particulate MH added.

a TA, ( b ) xCO 2 , and ( c ) pH timeseries observations upstream (blue) and downstream (red) of Mg(OH) 2 addition to wastewater (grey shaded areas). Squares in panel ( c ). represent discrete samples. Red and blue shading in ( b ). represent extrapolated xCO 2 values from pH observations in ( c ).

Ocean discharge of enhanced alkalinity wastewater

The signal from the addition of MH to effluent at HWTW was expected to arrive at the diffuser ~9 h after the start of dosing (supplementary Table S- 1 ). The periods before and when MH was expected to flow at the diffuser are hereafter referred to as ‘pre-MH’ and ‘during-MH’. Lower salinity water (<35.1) in the vicinity of the outfall had the characteristics of treated effluent from the HWTW: higher xCO 2 , TA, DIC, ammonia and optical backscatter (a measure of particulates), lower pH, and lower oxygen saturation. TA and DIC were elevated around the outfall by ~200 μmol/kg and ~100 μmol/kg respectively compared with long-term observations in the nearby coastal English Channel 19 . A significant correlation between ammonia and salinity was found for marine samples (Pearson R 2  = 0.68, p  < 0.001) with an intercept of 2316 ± 257 μmol/kg, comparable with measurements at HWTW (2774 ± 149 μmol/kg).

The carbonate system in seawater is constrained by a series of equilibria, such that two of its four variables [TA, DIC, pH, and pCO 2 , the partial pressure of CO 2 ] can predict the remaining two 20 . Here, we found agreement between in situ pH or pCO 2 with that predicted from the seawater carbonate equilibrium when DIC was used with pCO 2 or pH as predictors. However, the discrete TA/DIC pair was a poor predictor of pH/pCO 2 . When the average TA calculated from pCO 2 /DIC was subtracted from the average measured TA, we found an ‘excess alkalinity’ of 55.2 μmol/kg (TA measured  > TA calc ), likely reflecting the presence of organic alkalinity in wastewater (weak organic bases). While organic alkalinity contributes to TA in the short term, this is likely a transient effect (hours-days) as the remineralisation of organic bases would lead to both CO 2 production and a reduction in TA 21 . This transient contribution of organic alkalinity and its biogeochemical evolution (dilution, remineralisation) are beyond the scope of this paper, but worthy of further study in the context of OAE.

Diurnal TA variability close to the diffuser (up to 150 μmol/kg), was consistent with TA variability at HWTW (Fig.  2 ), thermal stratification as well as tidal- and wind-driven mixing and advection of seawater. Direct measurement of alkalinity enhancement at the marine discharge was confounded by rapid dilution of the wastewater which is illustrated by the salinity distribution with distance from the diffuser (Fig.  3a .). In order to constrain this mixing, we constructed a mixing model with two end-members: (a) final effluent with salinity 2.4 and (b) coastal seawater with salinity 35.18 (average salinity 300–350 m from diffuser). An exponential fit was used to describe the relationship between salinity and distance from the diffuser [Salinity = 2.4 + 32.7 × (1-e (-K1×D) ) + 0.08 × (1-e (-K2×D) )], where D is the distance from the diffuser, K 1  = 0.32 m −1 and K 2  = 0.01 m −1 are empirical constants which define how rapidly the end-members mix. Salinity observations were broadly constrained by K 1 in the range of 0.18–1.00 m −1 and K 2 in the range of 0–0.01 m −1 (Fig.  3a .). The salinity mixing curves from this model were used to constrain the extent of the ‘initial mixing zone’ (IMZ), the region where the final effluent rises under its own buoyancy 22 , 23 . We defined the edge of the IMZ as salinity = 35 psu. For K 1  = 0.18 m −1 and K 2  = 0.01 m −1 (higher mixing scenario), the outer edge of the IMZ was thereby calculated as 38 m (grey shaded area in Fig.  3 ). The model IMZ salinity was used to calculate the respective fractions of effluent/seawater and an average dilution factor of 260 within the IMZ (590 at the edge of the IMZ).

a Salinity against distance from the diffuser (black circles) with discrete samples from the 20th of September (orange circles). Salinity data were fitted with an exponential function (red line and shaded area) which were used to define the ‘initial mixing zone’ (grey shaded area). b Mean TA for pre-MH (light blue circle) and during-MH samples (dark blue square) from the 20th of September (error bars are standard deviation). TA pre MH (red curve) was derived from salinity in ( a ). and the linear regression of TA against salinity. The red shaded area corresponds to the range of mixing curves in ( a ). The purple curve represents a four-fold increase in MH addition.

Final effluent dilution was in agreement with the range of dilution factors from hydrodynamic modelling (400–2700) 24 , but also suggested that the alkalinity enhancement at HWTW would be practically undetectable in the IMZ. The 260-fold effluent dilution in the IMZ suggested that the alkalinization at HWTW (<943 μmol/kg) should be practically undetectable at sea (3.6 μmol/kg). A meaningful comparison of pre- and during MH TA could only be done for the 20th of September (logistical constraints on previous days resulted in the collection of a single during-MH sample). A during-MH TA increase of 33.8 ± 14.7 μmol/kg was measured (average during-MH TA = 2415.8 μmol/kg, compared to 2382.0 μmol/kg pre-MH). Closer examination of the data revealed a sampling bias whereby during-MH samples were collected closer to the diffuser compared to earlier samples which would result in higher TA regardless of MH addition. Salinity from the mixing model above was combined with the TA-salinity regression for pre-MH coastal and HWTW upstream discrete samples (TA pre MH  = 6038.9–103.857 × Salinity, R 2  = 1.00, n  = 18, p  < 0.001), to calculate the corresponding TA with distance from the diffuser (red line in Fig.  3b ). Both pre- and during-MH TA on the 20th of September fell within the envelope of the mixing model (Fig.  3b ). Similarly, DIC was related to salinity for pre-MH coastal and HWTW upstream discrete samples (DIC pre MH  = 6535.0–125.212 × Salinity, R 2  = 0.999, n  = 18, p  < 0.001) and the observed during-MH and pre-MH DIC difference (2154.7 μmol kg −1 ( n  = 8) and 2122.7 μmol kg −1 ( n  = 4) respectively) could be ascribed to dilution within the initial mixing zone. We therefore concluded that the observed TA and DIC increase could be ascribed to proximity to the diffuser rather than the addition of MH. These findings confirm the difficulty of measuring small TA changes against a high background at sea.

Nevertheless, we found other carbonate system effects, consistent with alkalinization. In the immediate vicinity of the diffuser (within 5 m) and at salinities <35.1 psu: (a) during-MH xCO 2 was lower by 27.2 μatm and (b) pH was higher by 0.017 units compared to earlier observations on the same day (Fig.  4 ). Both of these parameters were consistent with alkalinization of the effluent. Modest CO 2 reduction was thereby corroborated by independent measurements of xCO 2 and pH during this ‘proof of concept’ trial. Our observations demonstrate that xCO 2 and pH are more suitable parameters for OAE monitoring than TA, because of lower background values for xCO 2 and established sensor technologies that allow continuous observations rather than discrete sampling.

a xCO 2 against salinity within 350 m and ( b ) 5 m of the discharge of treated wastewater to the coastal ocean. c pH against salinity within 350 m and ( d ) 5 m of the discharge. Blue and red symbols denote pre-MH and during-MH observations respectively. Statistically significant differences were constrained to within 5 m of the discharge point at salinity <35.1 (vertical line).

Alongside deep and rapid cuts in CO 2 emissions, early CDR deployment will be required in order to limit global warming and maintain ocean health 25 . Here, the addition of MH achieved substantial wastewater alkalinity enhancement and CO 2 reduction with an efficiency of 0.73 mol CO 2 reduction per mol TA added. It is worth noting that downstream recovery to pre-addition conditions was rapid with no apparent ‘memory’ of the MH addition in either the TA, xCO 2 or the pH data. From an operational and regulatory perspective, this demonstrates a clear ‘exit strategy’ should the MH addition need to be stopped. Without addition of MH, the wastewater was supersaturated with respect to atmospheric CO 2 and this excess CO 2 (34× atmospheric equilibrium) would therefore be expected to degas to the atmosphere once discharged to the ocean. Post-discharge CO 2 reduction and pH increase, consistent with alkalinization, were confirmed, but direct detection of these was limited to within a few metres of the diffuser due to the modest addition of MH, primarily aimed at ‘proof-of-concept’.

The specific location of this trial maximises the CDR potential of alkalinization as the prevailing currents would maintain alkalinized water in contact with the atmosphere for ~4 years 26 . At the present site, upscaling the MH concentration would lead to complete depletion of the wastewater CO 2 and increase the final effluent pH. A four-fold increase in MH concentration would thereby increase ‘final effluent’ pH to ~8.46 (for a 0.08% MH % Volume Fraction ). In seawater, our mixing model shows that a four-fold increase in MH addition would raise TA by 19.5 μmol/kg at the edge of the IMZ (purple curve in Fig.  3b ). The mean area-weighted TA increase within the IMZ would be 49.0 μmol/kg. We expect such an increase in TA to raise pH by 0.11 units (to 8.20) and lower pCO 2 by 111 μatm (based on seacarb 3.3.1 27 , from 461 μatm). Using a gas transfer model 28 and average wind speed of 4.5 m/s, this pCO 2 reduction would switch the IMZ from a current source of CO 2 (efflux of 2.2 mmol/m −2 /d) to a sink for atmospheric CO 2 (influx of −3.7 mmol/m −2 /d). Seawater pH would thereby remain within both the regulatory limit of 8.5 and the seasonal envelope for these coastal waters (e.g., pH variability of 0.38 units) 19 . Within the IMZ, the mean TSS would increase by ~1.0 mg/L (260-fold dilution of 254 mg/L undissolved TSS in final effluent). Particulate TA, ejected from the diffuser, would reach the sea surface and behave as quasi-dissolved since the settling velocity (settling drag) is at least one order of magnitude lower than the horizontal velocities (drag) from wind and tides. Our calculations also show that a four-fold increase in seawater alkalinity enhancement would increase the aragonite saturation state (Ω Ar ) from 2.4 to 3.0, i.e., below the threshold of 7 where CaCO 3 precipitation has been reported which might render alkalinization ineffective for CDR 29 . We stress that lifecycle embedded and operational emissions exceeded the avoided emissions for the present trial due to its limited scale. However, our ‘proof of concept’ trial justifies further work exploring the efficacy of lower emission Mg(OH) 2 and field trials focused on increasing the scale of operations and resulting CDR. Some operational emissions will increase proportionately with upscaling such as those associated with the additional feedstock required. Others, such as transport, dosing, and monitoring setup, increase by a smaller proportion. These considerations are captured in published Monitoring Reporting and Verification protocols (MRV) 30 , 31 . Our simple mixing model illustrates the potential of a distributed network of sites, each with modest MH addition, to provide an effective solution as part of a wider climate change mitigation strategy 13 .

While this study demonstrates the CDR potential of alkalinity enhancement using MH and its potential to counteract ocean acidification, it is imperative to consider potential ecosystem impacts. Previous experimental exposure of the shore crab ( Carcinus maenas ) to alkali identified physiological responses at higher pH increases (+0.4 and +0.7 pH units) 32 than observed here or expected from a four-fold increase in MH concentration. Such pH perturbations in seawater are unrealistic for the purpose of CDR as they could lead to precipitation of CaCO 3 with no CDR benefit 29 . Average Mg(OH) 2 concentration in the IMZ would increase by 1.7 mg/L for a four-fold increase in MH addition to the final effluent. For reference, Mg(OH) 2 toxicity levels (LC50) for an aquatic invertebrate ( Daphnia magna ) and fish ( Pimephales promelas ) are 285 and 511 mg/L respectively 33 , 34 while magnesium ion LC50 is 1300 mg/L and 2100 mg/L respectively 35 . Based on this evidence, upscaling OAE by addition of Mg(OH) 2 to wastewater can provide a climate change mitigation solution while staying within the range of chemical conditions experienced by marine ecosystems.

Analytical methods

Discrete sample measurements for carbonate parameters were made at 21 ± 0.5 °C from the same subsample in triplicate as described below and in the order of CO 2(aq) , Dissolved Inorganic Carbon (DIC), pH, Total Alkalinity (TA). Sample handling and methodologies followed ‘best practice’ as recommended by Dickson (2007). CO 2(aq) was determined by sparging a known volume of subsample with N 2 gas (BOC Ltd.) followed by non-dispersive infrared detection of CO 2(g) (LiCor Biosciences, LI-7000). The LI-7000 instrument was calibrated with acidified Na 2 CO 3 standards (see next). DIC was determined by acidifying a known volume of subsample with 8.5% v/v H 3 PO 4 , followed by sparging with N 2(g) and non-dispersive infrared detection (LICOR; LI 7000) using an Apollo SciTech AS-C3 DIC analyser (Kitidis et al., 2017). The instrument was calibrated with Na 2 CO 3 standard solutions (range: 0–3.2 mmol/L Na 2 CO 3 ), [prepared by serial dilution of a 0.4 mol/L stock standard solution: 10.599 g of Na 2 CO 3 (VWR Chemicals, prod.no. 27767.295; Lot 21C154126), weighed on an ISO9001 calibrated microbalance (Ohaus Explorer Pro, s/n 1127033970) and dissolved in 250 mL of analytical grade water (Millipore, Milli-Q, 18.2 MOhm) which was previously sparged with N 2 gas (BOC Ltd.) in order to remove background CO 2(g) ]. Analytical precision for DIC was <2 μmol/kg. A certified reference material from the Andrew Dickson laboratory at Scripps Institute of Oceanography (batch # 202) with a reference DIC concentration of 2043.3 μmol/kg was analysed in triplicate and determined as 2038.1 ± 1.7 μmol/kg.

TA was determined by open cell, dynamic end-point, titration (Dickson, 2007) with 0.0396 mol/L HCl solution (diluted from 10 mol/L HCl, Merck ACS, prod.no. 30721, Lot STBG8596) and calibrated with Na 2 CO 3 standard solutions described above using an automated titration system (Metrohm, 916 Ti-Touch, s/n: 1916002032101) with a 20 mL dosing unit (Metrohm 800 Dosino, s/n: 1800003010533) and glass electrode (Metrohm iConnect 854; s/n: 00682610). The certified reference material from the Andrew Dickson laboratory (batch # 202) with a reference TA concentration of 2215.1 μmol/kg was analysed in triplicate and determined as 2220.7 ± 5.6 μmol/kg.

In the laboratory pH was determined with a glass electrode (Metrohm iConnect 854; s/n: 00682610). The instrument was calibrated with 3 NBS buffers at pH4 (Fisher, prod.no. J/2826/15, Lot 2034055), pH7 (Fisher, J/2855/15, Lot 2028757) and pH10 (VWR Chemicals, prod.no. 85044.001, Lot 211014118). The calibration slope was 99.8% of the idealised Nernst slope. Note that pH is reported on the NBS scale here (pH NBS ). Inorganic nutrients samples were collected by filtering 50 mL of effluent into acid cleaned high density polyethylene (HDPE) bottles (Whatman, WHA10404006, 0.45 µm cellulose acetate filter), and then frozen at −20 °C and transported back to the laboratory for analysis. The samples were defrosted according to GO-SHIP nutrient protocols 36 . Dissolved inorganic nutrient concentrations were determined by colorimetric analytical techniques using a 5-channel Bran and Luebbe segmented-flow autoanalyzer (Bran and Luebbe, model AA3). Standard analytical techniques were used for nitrate + nitrite (NO 3 − + NO 2 − ) 37 , nitrite (NO 2 − ) 38 , ammonia+ammonium (NH x ) 39 , phosphate (PO 4 3− ) 40 and silicate (SiO 2 − ) 40 . All sample handling and analysis was carried out according to GO-SHIP nutrient protocols.

Total Suspended Solids (TSS) were determined as dry weight per litre following the regulatory analytical protocol (ISBN 011751957X). A known volume of water was filtered through dry, pre-weighed glass fibre filters (1.2 μm nominal pore size; Whatman, GF/C), dried at 105 °C and reweighed (balance: Ohaus Explorer Pro, s/n 1127033970). The dry retentate was divided by the volume of sample filtered to derive TSS concentration. TSS samples were air-dried in the field (within 12 h of collection) prior to drying at 105 °C overnight at the PML laboratory (1–5 days post-collection).

Lab assessment of alkalinity enhancement in wastewater

Prior to the field trial, laboratory experiments were conducted in order to: (a) assess the impact of MH addition on the wastewater carbonate system, (b) determine field-trial dosing rates within the regulatory discharge framework (upper pH limit of 8.5 and TSS limit of 150 mg/L at the edge of the ‘initial mixing zone’ at sea) and (c) assess the potential formation of struvite (NH 4 MgPO 4 ·6H 2 O) and vivianite (Fe 3 (PO 4 ) 2 ), precipitates that could consume MH and thereby compete with CO 2 as well as cause operational disruption of the wastewater treatment plant (WTW). Wastewater from Hayle WTW (HWTW) was mixed with a commercial MH suspension (TIMAB Magnesium; TIMAB 53 S mare). Circa 990 mL of wastewater were transferred to a 1 L volumetric cylinder before pipetting the corresponding volume of MH suspension and adding more wastewater up to the 1 L mark. The volumetric cylinder was inverted and the solution was gently transferred to 0.5 L glass bottles with a ground neck and stopper. Particular care was taken during the transfer in order to avoid turbulent mixing and bubble formation as well as headspace formation in the incubation bottle.

Field trial setup - Hayle wastewater treatment works

MH suspension (TIMAB magnesium, TIMAB 53 S mare, magnesite) was added to ‘final effluent’ at the Hayle wastewater treatment works (HWTW) on three consecutive days for ca 8.5 h on the 18th, 19th, and 20th of September 2022. The start times, pipe volume, and flow data provided by South West Water Ltd. were used to calculate the expected time of MH arrival at the diffuser outside St. Ives Bay (supplementary Table S- 2 ). MH suspension was supplied in four intermediate bulk containers (IBC), agitated with an industrial IBC mixer (Axesspack, TM IBC-169) and delivered to the effluent at a constant rate of 3.5 L/min using an industrial slurry dosing pump (Verderflex, iDura 15). In total, 4 tonnes of MH suspension were added to the final effluent.

Monitoring stations were established upstream (50.171846°N, 5.434764°W) and downstream (50.171283°N, 5.436859°W) of the MH addition and were instrumented with CO 2 and pH sensors with additional discrete sampling for DIC, TA and pH, nutrients and TSS (Fig.  5 ). Two custom-made, autonomous sampling systems were installed at the wastewater treatment works on 15th September 2022. Water was drawn from a sump at the upstream and downstream stations using a custom-made, solar-powered pump system (Blackfish Engineering, Bristol, UK). Each system comprised a diaphragm pump (Flojet, RLFP 222202D), two 400 W solar panels, 440 Ah battery capacity, timer, and associated electronic circuitry. Pumped effluent was directed to a flow-through manifold housing a CO 2 sensor (ProOceanus CO2 Mini), pH sensor, and logger (Hobo MX2501 pH and Temp). The CO 2 sensors were factory calibrated and the pH sensors were calibrated prior to deployment using NBS buffers. Wastewater flow through the sensor manifold was regulated to 2 L/min (manufacturer recommends 1–3 L/min for the CO2 Mini). Both pump systems were operated on a timer, switching on/off every 15 min, apart from the MH addition periods when the pumps were operating continuously. The outlet of the flow-through manifold returned wastewater to the sampling sump and was used for discrete sample collection. Both CO 2 Mini sensors unexpectedly stopped on the 20th of September at 14:33 BST (at the end of the last MH addition). Subsequent CO 2 data were inferred from the CO 2 -pH relationship prior to this point (CO 2  = 21521997120 × e −3.22pH ). propagating a pH uncertainty of 0.05 units (shaded area in Fig.  2 ). Discrete samples were collected for analysis of carbonate system parameters (DIC, TA, pH), nutrients and TSS following the protocols described above.

a Study area and location of the MH addition field trial in north Cornwall, UK (red square). The general residual circulation follows the black arrows, flowing North from the study area, clockwise around Great Britain and counter-clockwise around the North Sea. b Locations of the Hayle Wastewater Treatment Works (HWTW) where MH suspension was added to final effluent and diffuser at the end of the discharge pipe for final effluent (black square). The inset shows the survey tracks around the diffuser (black square) and extent of the initial mixing zone (red shaded area).

Field trial setup - Coastal ocean survey

Sampling at the diffuser outside St. Ives Bay took place on a 20 m long survey vessel and a 7 m long fishing vessel (static vessel) over the period 18–21 September 2022. The static vessel assumed a position at the diffuser site while the survey vessel carried out spatial survey work consisting of a series of stations and drifts (Fig.  5 ). In calm conditions and slack water during the tidal cycle, the diffuser was identifiable at the surface as four clear patches suggesting four diffusers spaced fifteen metres apart and in line with the known position of the outflow pipe on land. The northernmost diffuser is located at approximately 23 m depth at 50.23885°N, −5.424404°E with the other diffusers on a bearing of 117 o . Ship positions were logged from the navigational instruments of both vessels at half-hourly intervals and at the start/end of stations/drifts. These data were interpolated onto the sampling times and their horizontal distance from the northernmost diffuser calculated using the Haversine formula.

The static vessel was equipped with a Conductivity Temperature Depth (CTD; RBR Concerto3, s/n 60614) and CO 2 sensor package (ProOceanus CO 2 -PRO) which was deployed at anchor, primarily centred on the second diffuser from North. A 12 V submersible pump (Seabird) was used to maintain a constant flow across the CO 2 sensor head, powered by a 78 Ah battery pack. Data were logged internally and downloaded daily. The sensor package was deployed at constant depths (either at 17 m or 10 m) for several hours at a time.

The survey vessel was equipped with a multiparameter CTD (RBR Maestro3, s/n 209876) with additional backscatter, dissolved oxygen and pH sensors (pH was factory calibrated and checked against the same buffers as the lab electrode – no recalibration required). A 28 m flexible hose (10 mm id) was attached to the instrument wire with the inlet co-located with the CTD. This setup was used in ‘profiling’ mode, drawing water from the CTD depth. Water was drawn on deck via a diaphragm pump (Flojet, RLFP 222202D) and directed to a flow-through manifold housing a CO 2 sensor (ProOceanus CO 2 -PRO CV), pH sensor (Sunburst Technologies, SAMI-pH, s/n 0002) and conductivity probe (RS Components, RS PRO 205-0958). This setup was chosen to facilitate power independence (the setup was 12 V battery powered) and quasi real-time data monitoring with a hysteresis of 28 s (the residence time of the sampling hose). The CO 2 sensor was factory calibrated and the pH sensor was calibrated using NBS buffers prior to deployment and following the manufacturer’s instructions. The flow through the sensor manifold was regulated to 2 L/min. Discrete samples were collected from the flow-through manifold outflow for analysis of carbonate system parameters (DIC, TA, pH), nutrients, and TSS following the protocols described above (supplementary Table S- 4 ). All instruments were synchronised with the clock of the same PC and GMT to facilitate data merging.

Marine xCO2 data processing

CTD, CO 2, and pH sensor data were merged giving a combined 2 Hz dataset (CTD logging frequency was 8 Hz which was averaged to 2 Hz, interpolating the 1 s CO 2 data and 1 min SAMI-pH data). A 28 s offset was applied to the survey vessel CO 2 and pH data in order to account for the residence time of the sampling hose. The merged dataset was filtered to exclude: (a) all data where the timestamp difference between the CTD and CO 2 -PRO CV exceeded 1 s (this removed instances where either instrument was not operational); (b) all data where the salinity was <33 (spurious singular data points in time against a background of salinity >33); (c) a station on the 19th September when instruments were not operating properly (constant values for all parameters apart from time and depth); (d) all data in the uppermost 20 cm of the water column (mostly data at the start of a profile); (e) all data in the uppermost 1 m of the water column with salinity <34.62 (spurious singular data points in time; similar to b and d).

The filtered merged dataset comprised 97,566 data points from the survey vessel and a further 62,268 from the static vessel.

During deployment, it became apparent that the CTD (and hose inlet) drifted into and out of the outfall plume for short periods (typically seconds to <1 min). These events manifest themselves as salinity excursions (>33 salinity) and further corroborated by elevated backscatter compared to background seawater (ca. 35.2 salinity). It appears likely that the turbulent flow directly above the diffuser diverted the CTD package outside of the effluent plume, inhibiting observations for longer periods. During these salinity excursion events CO 2 increased as expected, but was lagging behind salinity (Fig.  6 ). Salinity would thereby decrease, but CO 2 would not rise concurrently and often peak after the salinity excursion event (see xCO 2 raw in Fig.  6 ). This resulted in a salinity-CO 2 mismatch likely caused by the relatively slow diffusion of CO 2 across the gas-permeable sensor membrane. The diffusion step results in a t63 response time of 50 s (the time it takes for the sensor to reach 63% of full response to a signal). The data were therefore scaled by the 1st and 2nd derivative of CO 2 over 3 s to obtain CO 2 scaled . This calculation improved the matchup between salinity excursion events and concurrent increase in CO 2 , but it also amplified the natural ‘noise’ of the CO 2 sensor data (see xCO 2 scaled in Fig.  6 ). Note that the equation below refers to xCO 2 for technical consistency, where ‘x’ marks the mixing ratio (in ppmv) rather than the partial pressure (in μatm). The scaling equation was empirically optimised to match calculated xCO 2 from discrete TA/DIC samples:

A salinity excursion event on the 19th of September 2022 showing the time-lag in concurrent CO2-PRO CV data (xCO 2 raw) and reduced time-lag for scaled data (xCO 2 scaled). Note that the scaling applied here improves the time-lag between low salinity events and xCO 2 , although some hysteresis remains.

Marine pH data processing

The spectrophotometric instrument reports pH on the total scale at a constant salinity (pH SAMI-35 ). These data were corrected (pH SAMI in situ) to in situ salinity 41 and further corrected to in situ temperature using the pHinsi function in the R-package seacarb with the constants of ref. 42 , Dickson 1990 43 , Perez and Fraga, 1987 44 and Upströhm 1974 45 (R-version 4.0.1; https://www.r-project.org/ ). Both pH SAMI in situ and measurements using the glass electrode on the RBR instrument (pH RBR ) were converted to the free scale using the pHconv function in the R-package seacarb .

Marine carbonate system quality control and internal consistency

pH SAMI in situ was significantly correlated with measurements using the glass electrode on the RBR instrument (pH RBR ) with a residual standard error of 0.008 units (Pearson R 2  = 1.00, p  < 0.001) (Fig.  7 ). We used the timestamp of discrete samples to find the nearest corresponding pH RBR and pH SAMI in situ. pH RBR was in agreement with pH calc (pCO2/DIC) ; calculated pH from the discrete DIC data and corresponding CO 2 partial pressure (pCO 2 ; calculated from xCO 2 using the x2pCO2 function in the R-package seacarb ) (Fig.  7 ). This agreement between independent instruments gives high confidence to the pH data as well as xCO 2 and DIC data. However, pH calculated from discrete TA/DIC (pH calc (TA/DIC) ; calculated using the carb function in the R-package seacarb ) did not agree with pH RBR , pH SAMI in situ or pH calc (pCO2/DIC) (Fig.  7 ). pCO 2 from DIC/pH was in agreement with measurements of pCO 2 with a standard error of 10.5 μatm (Pearson R 2  = 1.00, p  < 0.001) (Fig.  7 ). When the average TA calculated from pCO 2 /DIC was subtracted from the average measured TA, we found an ‘excess alkalinity’ of 55.2 μmol/kg (55.6 μmol/kg when TA is calculated from DIC/pH).

a pH from the SAMI instrument and calculated from pCO 2 /DIC and TA/DIC against pH from the RBR instrument. b Measured pCO 2 against calculated pCO 2 from TA/pH.

Final effluent – seawater dilution

Salinity in the vicinity of the diffuser (e.g., within 25 m) was highly variable (range: 33.1–35.3) reflecting extensive and dynamic mixing between final effluent and coastal seawater (Fig.  3a ). In contrast, salinity in the zone 300–350 m away from the diffuser was higher on average and much less variable with a mean ± standard deviation of 35.183 ± 0.005 psu (Fig.  3a ). We defined two end-member mixing between: (a) final effluent with salinity 2.4 and (b) coastal seawater with salinity 35.183. An exponential fit (Eq.  2 ) was used to describe the relationship between salinity (S) and distance (D) from the diffuser (red curve in Fig.  3a ):

where K 1 and K 2 are empirical constants which define how rapidly the end-members mix. High- and low-mixing variants of Eq.  2 were used to constrain the field-observations (red shaded area in Fig.  3 ). The mixing curves defined by K 1 and K 2 where used to constrain the extent of the ‘initial mixing zone’ by specifying a salinity of 35.00 psu at its outer edge under the low mixing scenario (i.e., within 0.18 psu of the seawater end-member). The outer edge of the initial mixing zone was thereby calculated as 38 m (grey shaded area in Fig.  3 ).

In order to calculate the dilution of final effluent by coastal seawater, we defined two end-members: (a) final effluent with salinity 2.4 and (b) coastal seawater with salinity 35.183. For any sample point, its salinity is given by Eq.  3 :

where Salinity FE and Salinity sea are the respective salinities of the final effluent (2.4 psu) and coastal seawater (35.18 psu). f FE and f sea are the fraction of final effluent fraction of seawater with which it mixes. Since the salinities of the samples and end-members were known, we solved Eq.  2 for f FE given that f FE +f sea  = 1. The final effluent dilution factor (DF) is then given by Eq.  4 :

The average dilution factor and the respective value at the edge of the initial mixing zone were calculated as 260 and 590.

At the point of discharge, the flow velocity would dissipate rapidly. Assuming that the 0.270 m 3 /s flow is divided equally between the four diffusers, each with a diameter of 60 cm, we calculate a mean upwards velocity of 24 cm/s from each diffuser. If we assume that this velocity dissipates with increasing volume in the IMZ cone, then the effluent would require ~3 min to reach the surface.

As an independent coherence test we calculated the mean current in the initial mixing zone. f FE and f sea can be replaced by the final effluent flow (0.270 m 3 /s) and a theoretical seawater flow, Flow sea (units: m 3 /s) in Eq.  3 . Flow sea can then be calculated iteratively by varying Flow sea to minimise the absolute difference between measured and calculated salinity using Eq.  3 (replacing f FE with a flow of 0.270 m 3 /s while f sea becomes Flow sea ). We assumed that Flow sea is flowing across the wastewater plume represented as the orthographic projection of a cone on the vertical water column plane, i.e., an inverted triangle with its apex at the diffuser, a height of 23 m (the water depth at the diffuser), spreading to a width (base of the triangle) of 2 × 38 m at the surface (plume-plane area A = (23 × (2 × 38))/2 m 2 ). The mean current is thereby calculated as Flow sea divided by the plume-plane area (units: m 3 /s divided by m 2  = m/s). This calculation gave a mean current of 0.16 knots which is within the 0.1–0.4 knots neap-tide currents for the area 46 .

Magnesium hydroxide dissolution and upscaling

The lab experiment and field trial TSS data showed complete dissolution of MH at volumetric mixing ratios up to 0.021% v/v MH in wastewater. For, the 0.1% and 1% v/v ratios in the lab experiment, the respective TSS 18h concentrations, i.e., undissolved MH, were 390 and 5314 mg/L above the background TSS in raw effluent (supplementary Table S- 1 ). In order to quantify TSS dissolution we fitted an exponential asymptotic curve to the lab-experiment data: ΔTSS = 869.2 × (1-e (0.0005×TSSin) ), where ΔTSS = TSS in -TSS 18h (Fig.  8 ). ΔTSS was assumed to represent dissolved MH.

Dissolution of Total Suspended Solids (ΔTSS) against initial TSS (TSS in ) from lab- and field trial data. An exponential asymptotic curve was fitted to the lab-experiment data (solid curve). The red square represents a hypothetical four-fold upscaling of MH addition from the field trial (to 0.08% v/v MH suspension in wastewater).

The TSS dissolution fit above, was used to calculate dissolved and particulate MH loads under a hypothetical four-fold increase in MH dosing to 0.08% v/v MH addition to wastewater. Under this scenario, the concentration of TSS in at the point of MH addition would be 418 mg/L 53% solids/L MH of which 98.5% is Mg(OH) 2  = 522 g/L Mg(OH) 2 in the pure MH suspension applied as 0.08/100 to wastewater = 418 mg/L Mg(OH) 2 . Of the 418 mg/L TSS in , our ΔTSS fit predicts dissolution of 164 mg/L, leaving 254 mg/L of undissolved TSS in the final effluent. Given a dilution factor of 260 for the final effluent in the IMZ, we would expect the mean IMZ TSS to be ~1.0 mg/L higher. Assuming that the 164 mg/L dissolved TSS represents MH, we calculate an increase of 2.8 mmol/kg Mg(OH) 2 or 5.6 mmol/kg TA (164 mg/L ÷ 58.3 g/mol and 1 mmol Mg(OH) 2  = 2 mmol TA). The 254 mg/L of undissolved TSS equates to 4.4 mmol/kg Mg(OH) 2 or 8.7 mmol/kg particulate TA. The total (dissolved+ particulate) TA increase in final effluent would thereby be 14.3 mmol/kg.

The dilution of a 14.3 mmol/kg TA addition in coastal seawater is shown as the purple curve in Fig.  3b . This mixing curve, implicitly assumes that the TA addition is in the dissolved or quasi-dissolved phase which is further discussed below. In the initial mixing zone, we would therefore expect a mean TA increase of 55 µmol/kg (260-fold dilution). The 14.3 mmol/kg MH addition to wastewater equals 418 mg/L Mg(OH) 2 , or 152 mg/L Mg 2+ . Within the IMZ, we would therefore expect average Mg 2+ to increase by 0.6 mg/L. In order to examine the behavious of the undissolved MH, we used Stokes Law to calculate the settling velocity (w) of a typical MH particle:

where ρ p and ρ f are the density of the particle (2300 kg/m 3 ) and fluid (1000.3 kg/m 3 for wastewater; 1025.8 kg/m 3 for seawater), g is the acceleration of gravity (9.81 m/s), r is the particle radius (1 μm) and µ is the dynamic visosity of the fluid (0.001 kg/m*s). The settling velocity of a particle was thereby calculated as 1.0 cm/h in both wastewater and seawater. Within the 1 m diameter pipe, the mean flow of 0.270 m 3 /s, gives a horizontal velocity of 1.2 km/h. The horizontal velocity is therefore >10 5 times greater than the settling velocity. Since the drag force is proportional to the flow velocity, the horizontal drag force is more than 100 thousand times greater than the settling drag force. It is therefore highly unlikely that particles would settle in the pipework from HWTW to the coastal ocean.

At the point of discharge, the flow velocity would dissipate rapidly. Assuming that the 0.270 m 3 /s flow is divided equally between the four diffusers, each with a diameter of 60 cm, we calculate a mean upwards velocity of 24 cm/s from each diffuser. If we assume that this velocity dissipates with increasing volume in the IMZ cone, then the upwards velocity within 1 cm of the surface is in excess of the settling velocity, i.e., particles ejected from the diffuser would reach the sea surface. In seawater, these particles would then take 96 days to settle to the sea floor assuming no further turbulence (23 m divided by 1 cm/h). The MH particles would therefore reach the surface and sink, but at the same time dissolve and convert carbonic acid, from dissolved CO 2 , to bicarbonate ions. The reduction in seawater dissolved CO 2 (carbonic acid) would then favour influx of CO 2 from the atmosphere. Tidal currents in this region are in the range of 5–20 cm/h 46 , i.e., the horizontal drag on particles is one order of magnitude greater than the settling drag force. Tidal and wind-driven turbulence in this dynamic shelf-sea environment would therefore likely resuspend particulate TA rather than allow it to settle on the seafloor. This supports the underlying assumption in our mixing model that particulate TA (MH) would practically behave in a quasi-dissolved manner under hypothetical four-fold upscaling (purple curve in Fig.  3b ).

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Acknowledgements

The authors disclose support for the research of this work from the UK Department of Business Energy and Industrial Strategy [Contract PLA202081]. This synthesis was supported by UK NERC funding (CLASS Theme1.2, NE/R015953/1).

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Contributions

Authors V.K., S.A.R., W.J.B., G.H.R., S.F., G.T., and T.F. contributed to the study design, data synthesis, and interpretation. V.K., W.J.B., S.F., M.T., and G.T. contributed to fieldwork. V.K., S.F., M.T., G.T., E.M.S.W., and C.H. contributed to analytical work.

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Correspondence to Vassilis Kitidis .

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The authors declare that Planetary Technologies (authors S.A.R., W.J.B., G.H.R.) are developing commercial ocean CDR technologies. PML Applications (V.K., S.F., M.T., G.T., E.M.S.W., C.H., T.F.) provided independent and impartial scientific consultancy services to Planetary Technologies as the commercial subsidiary of Plymouth Marine Laboratory (PML).

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Kitidis, V., Rackley, S.A., Burt, W.J. et al. Magnesium hydroxide addition reduces aqueous carbon dioxide in wastewater discharged to the ocean. Commun Earth Environ 5 , 354 (2024). https://doi.org/10.1038/s43247-024-01506-4

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Received : 02 September 2023

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Published : 28 June 2024

DOI : https://doi.org/10.1038/s43247-024-01506-4

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• Drugs & Medications
• Gentle Laxative (Magnesium Hydroxide) 400 Mg/5 Ml Oral Suspension Hypertonic Laxatives

Gentle Laxative (Magnesium Hydroxide) 400 Mg/5 Ml Oral Suspension Hypertonic Laxatives - Uses, Side Effects, and More

Generic name(s): magnesium hydroxide, side effects, precautions, interactions.

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This medication is used for a short time to treat occasional constipation . It is a laxative (osmotic-type) that is thought to work by drawing water into the intestines , an effect that helps to cause movement of the intestines .This medication is also used to treat symptoms caused by too much stomach acid such as heartburn , upset stomach , or indigestion . It is an antacid that works by lowering the amount of acid in the stomach .

How to use Gentle Laxative (Magnesium Hydroxide) 400 Mg/5 Ml Oral Suspension Hypertonic Laxatives

Take this product by mouth as directed. For the chewable form, chew thoroughly before swallowing. For the liquid form, shake the bottle well before each dose. Carefully measure the dose using a special measuring device/spoon. Do not use a household spoon because you may not get the correct dose. If you are taking this medication for constipation , drink a full glass of water (8 ounces or 240 milliliters) with each dose. Follow all directions on the product package, or use as directed by your doctor. If you have any questions, ask your doctor or pharmacist .

Dosage is based on your medical condition and response to treatment.

Extended use or overuse of this medication for constipation may result in dependence on laxatives and ongoing constipation. Overuse may also cause diarrhea that doesn't stop, dehydration , and mineral imbalances (such as high magnesium ).

Diarrhea may occur. If this effect lasts or gets worse, tell your doctor or pharmacist promptly.

If your doctor has directed you to use this medication , remember that your doctor has judged that the benefit to you is greater than the risk of side effects. Many people using this medication do not have serious side effects.

Tell your doctor right away if you have any serious side effects, including: symptoms of high magnesium levels (such as muscle weakness , slow/irregular heartbeat, slow/shallow breathing, mental/mood changes such as confusion), symptoms of dehydration (such as decreased urination, dizziness , extreme thirst, very dry mouth ), stomach / abdominal pain , bloody stools , rectal bleeding .

A very serious allergic reaction to this drug is rare. However, get medical help right away if you notice any symptoms of a serious allergic reaction , including: rash , itching /swelling (especially of the face/ tongue /throat), severe dizziness, trouble breathing .

This is not a complete list of possible side effects. If you notice other effects not listed above, contact your doctor or pharmacist.

In the US -

Call your doctor for medical advice about side effects. You may report side effects to FDA at 1-800-FDA-1088 or at www.fda.gov/medwatch.

Before taking magnesium hydroxide, tell your doctor or pharmacist if you are allergic to it; or if you have any other allergies . This product may contain inactive ingredients, which can cause allergic reactions or other problems. Talk to your pharmacist for more details.

Before using this medication , tell your doctor or pharmacist your medical history, especially of: kidney disease , appendicitis or symptoms of appendicitis (such as stomach / abdominal pain , nausea / vomiting ), magnesium-restricted diet, sudden change in bowel habits that lasts for longer than 2 weeks.

During pregnancy, this medication should be used only when clearly needed. Discuss the risks and benefits with your doctor.

Drug interactions may change how your medications work or increase your risk for serious side effects. This document does not contain all possible drug interactions. Keep a list of all the products you use (including prescription/nonprescription drugs and herbal products) and share it with your doctor and pharmacist . Do not start, stop, or change the dosage of any medicines without your doctor's approval.

Some products that may interact with this drug include: raltegravir, sodium polystyrene sulfonate.

Keep all medical and lab appointments.

Lifestyle changes such as regular exercise and diet changes (including drinking enough water, eating a proper diet with fiber-rich foods such as bran, fresh fruits/vegetables) may prevent or relieve constipation .

Lifestyle changes such as stress reduction programs, stopping smoking , limiting alcohol, and diet changes (such as avoiding caffeine /certain spices) may help to reduce heartburn and other stomach acid problems.

Store at room temperature away from light and moisture. Do not store in the bathroom. Keep all medications away from children and pets.

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Making magnesium carbonate: the formation of an insoluble salt in water

In association with Nuffield Foundation

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When solutions of two soluble salts are mixed, a solid may form. The solid is called a precipitate, and the reaction is called a precipitation reaction. Precipitation reactions are used to make insoluble salts

In this experiment the soluble salts are magnesium sulfate and sodium carbonate, and the insoluble salt formed is magnesium carbonate, which can be filtered, dried and collected.

This is a short standard class experiment. It should take no more than 20 minutes to the point at which the wet product can be set aside to dry.

If the solutions can be provided in pre-measured 25 cm 3  quantities in labelled containers, distribution of chemicals and control of quantities can be easily managed, and the practical work can begin without delay.

Sodium carbonate in dilute solution is weakly alkaline. So the few other safety issues are essentially restricted to safe handling of glassware. Even these can be minimised by the use of polythene filter funnels. This experiment is therefore suitable as a class experiment for most classes.

• Eye protection
• Conical flasks (100 cm 3 ) x2
• Filter funnel (65 mm diameter or similar, note 1)
• Filter papers (size suited to funnels used)

Apparatus notes

• Polythene filter funnels are safer and cheaper than glass funnels. The size of filter paper, when folded, should match the funnel size. The cheapest grade of filter paper is okay for this experiment.
• Sodium carbonate solution, 0.5 M, 25 cm 3
• Magnesium sulfate solution, 0.5 M, 25 cm 3

Health, safety and technical notes

• Read our standard health and safety guidance
• Wear eye protection. 
• If the reagent solutions can be distributed in pre-measured quantities, waste is reduced and lesson organisation is easier. All containers used for these solutions should be labelled.
• Sodium carbonate solution, Na 2 CO 3 (aq) – see CLEAPSS Hazcard HC095a and CLEAPSS Recipe Book RB080.
• Magnesium sulfate solution, MgSO 4 (aq) – see CLEAPSS Hazcard HC059b .  
• Magnesium carbonate, 3MgCO 3 .Mg(OH) 2 .3H 2 O(s) – see CLEAPSS Hazcard HC059b . 
• Mix 25 cm 3  of magnesium sulfate solution and 25 cm 3  of sodium carbonate solution in a conical flask.
• Place the filter funnel in the neck of another conical flask.
• Fold the filter paper to fit the filter funnel, and put it in place.
• Swirl the reaction mixture gently, and pour a little at a time into the filter paper in the funnel. Only pour in enough solution at a time to leave the solution level 1 cm below the rim of the filter paper. Allow to filter through.
• A clear solution should collect in the flask. If the solution is not clear, and white cloudiness remains in it, you will need to repeat the filtration.
• Remove the wet filter paper carefully from the funnel and place on a clean dry paper towel. Label with your name(s) and leave in a warm place, safe from interference, until it has dried completely (a few hours).

Source: Royal Society of Chemistry

The apparatus set-up for the experiment making magnesium carbonate

Teaching notes

There are no significant hazards in this experiment, except for the risk of broken glass if a flask is knocked over.

The formation of precipitates on mixing two solutions is met frequently in chemistry. This experiment is intended as a first introduction to this phenomenon for 11–14 year olds, as well as to practical filtration techniques. The experiment can be made more exciting visually by making a coloured salt such as copper(II) carbonate; in this case the chemical hazard level is slightly higher, since copper(II) carbonate is HARMFUL.

Because this is intended as a first introduction, the interpretation should be restricted to developing the word equation as a summary of what has happened:

magnesium sulfate + sodium carbonate → magnesium carbonate + sodium sulfate

Suggesting the name of the salt left in solution is not easy for students at this stage. It needs to be approached carefully, probably by group or whole class discussion. Using cut-out card labels: ‘sodium’, ‘magnesium’, ‘carbonate’ and ‘sulfate’ for students to move around will help many of them grasp the idea of ‘swapping partners’.

You could add some interest to which salts are used, and which salts are formed. Mention their uses, if this helps the class to see that these substances are not just important in the laboratory. See below.

Background information

Magnesium sulfate is known as Epsom salts. This is because the water found at the spa at Epsom in Surrey contains this salt in quite high concentration. Epsom salts are rarely used nowadays, but were used in medicine as a purgative.

Sodium carbonate is found naturally in high concentrations in the soda lakes of Kenya and Tanzania in East Africa. It is also manufactured in vast quantities and used in many different industries, including the chemical industry itself and in glass making. It is found in the home as washing soda, and in some detergent powders. For more about sodium carbonate in general, you may be interest in the Royal Society of Chemistry Book,  Sodium carbonate: a versatile material .

Magnesium carbonate is found in the mineral dolomite, mixed with calcium carbonate. Most limestones contains a proportion of magnesium carbonate – some a very high proportion. Magnesium carbonate is used in industry as a major source of magnesium compounds, it is used in many medical preparations to treat indigestion and it is also used as gym chalk.

Sodium sulfate, known as Glauber’s salt, is found (like Epsom salts) in some natural brines. It is used in large quantities in industries such as wood pulp production, glass-making, and detergents, and is also as a mild laxative.

If this experiment is used with older students, you can ask them to work out the symbol equation:

MgSO 4 (aq) + Na 2 CO 3 (aq) → MgCO 3 (s) + Na 2 SO 4 (aq)

The ionic equation, together with the concept of ‘spectator ions’, is likely to be appropriate for fewer students. However, this is not likely to be the experiment where the concept of spectator ions is introduced, as there are better examples, with visual colour clues to what is happening. The ionic equation is:

Mg 2+ (aq) + CO 3 2- (aq) → MgCO 3 (s)

and the spectator ions are: Na + (aq) and SO 4 2- (aq)

Student questions

Here are some possible questions for students.

• What did you see happen in the flask when the solutions mixed?
• What has collected in the filter paper? Describe what you can see.
• What is the name of the solid salt you have made?
• Suggest the name of the salt left in solution in the flask at the end. Explain how you decided on this name.
• Complete the word equation for this reaction: magnesium sulfate + sodium carbonate → … + …

Additional information

This is a resource from the  Practical Chemistry project , developed by the Nuffield Foundation and the Royal Society of Chemistry.

Practical Chemistry activities accompany  Practical Physics  and  Practical Biology . 

© Nuffield Foundation and the Royal Society of Chemistry

• 11-14 years
• 14-16 years
• Practical experiments
• Reactions and synthesis

Specification

• AT.4 Safe use of a range of equipment to purify and/or separate chemical mixtures including evaporation, filtration, crystallisation, chromatography and distillation.
• AT4 Safe use of a range of equipment to purify and/or separate chemical mixtures including evaporation, filtration, crystallisation, chromatography and distillation.
• 3.21 Describe the method used to prepare a pure, dry sample of an insoluble salt
• 7 Production of pure dry sample of an insoluble and soluble salt
• C4 Production of pure dry sample of an insoluble and soluble salt
• preparation of insoluble salts by precipitation
• Precipitation is the reaction of two solutions to form an insoluble salt called a precipitate.
• Information on the solubility of compounds can be used to predict when a precipitate will form.
• The formation of a precipitate can be used to identify the presence of a particular ion.
• (o) the preparation of insoluble salts by precipitation reactions
• (i) atoms/molecules in mixtures not being chemically joined and mixtures being easily separated by physical processes such as filtration, evaporation, chromatography and distillation
• 2. Develop and use models to describe the nature of matter; demonstrate how they provide a simple way to to account for the conservation of mass, changes of state, physical change, chemical change, mixtures, and their separation.
• 4. Classify substances as elements, compounds, mixtures, metals, non-metals, solids, liquids, gases and solutions.

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