Design Tool - Base Addition for ERD - Revision history

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 *[[Bioremediation_-_Anaerobic | Anaerobic Bioremediation]]
 *[[Chlorinated Solvents]]
 *[[pH Buffering in Aquifers]]

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 '''Related Article(s):'''
 *[[pH Buffering in Aquifers]]
 *[[Low pH Inhibition of Reductive Dechlorination]]

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	Acids are produced during Enhanced Reductive Dechlorination (ERD) which can cause pH to drop, inhibiting treatment. Alkaline materials including hydroxides and carbonates are sometimes added to the aquifer during ERD to maintain a pH of greater than 6, improving bioremediation performance. This article describes a simplified approach and a spreadsheet-based <u>[//www.enviro.wiki/images/b/b2/Base_Add_Design_Tool_-_Dec_2018.xlsx design tool]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion. To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.		Acids are produced during Enhanced Reductive Dechlorination (ERD) which can cause pH to drop, inhibiting treatment. Alkaline materials including hydroxides and carbonates are sometimes added to the aquifer during ERD to maintain a pH of greater than 6, improving bioremediation performance. This article describes a simplified approach and a spreadsheet-based <u>[//www.enviro.wiki/images/b/b2/Base_Add_Design_Tool_-_Dec_2018.xlsx design tool]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion. To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.
	<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>		<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>

← Older revision		Revision as of 01:55, 28 April 2022
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	Acids are produced during Enhanced Reductive Dechlorination (ERD) which can cause pH to drop, inhibiting treatment. Alkaline materials including hydroxides and carbonates are sometimes added to the aquifer during ERD to maintain a pH of greater than 6, improving bioremediation performance. This article describes a simplified approach and a spreadsheet-based <u>[//www.enviro.wiki/images/b/b2/Base_Add_Design_Tool_-_Dec_2018.xlsx design tool]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion. To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.		Acids are produced during Enhanced Reductive Dechlorination (ERD) which can cause pH to drop, inhibiting treatment. Alkaline materials including hydroxides and carbonates are sometimes added to the aquifer during ERD to maintain a pH of greater than 6, improving bioremediation performance. This article describes a simplified approach and a spreadsheet-based <u>[//www.enviro.wiki/images/b/b2/Base_Add_Design_Tool_-_Dec_2018.xlsx design tool]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion. To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.
		+	<div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
	'''Related Article(s):'''		'''Related Article(s):'''

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 '''Key Resource(s):'''
 *Spreadsheet-based <u>[[Media:Base Add Design Tool - Dec 2018.xlsx|Design Tool -- Base Addition for ERD]]</u>
-*[[media:2017-Borden-Post-Remediation_Evaluation_of_EVO_Treatment.pdf| Post-Remediation Evaluation of EVO Treatment: How Can we improve performance]]<ref name = "Borden2017EVO">Borden, R.C., 2017. Post-Remediation Evaluation of EVO Treatment: How Can We Improve Performance. Environmental Security Technology Certification Program, Alexandria, VA. ER-201581[[media:2017-Borden-Post-Remediation_Evaluation_of_EVO_Treatment.pdf| Report.pdf]]</ref>
+*[[media:2017-Borden-Post-Remediation_Evaluation_of_EVO_Treatment.pdf| Post-Remediation Evaluation of EVO Treatment: How Can we improve performance]]<ref name = "Borden2017EVO">Borden, R.C., 2017. Post-Remediation Evaluation of EVO Treatment: How Can We Improve Performance. Environmental Security Technology Certification Program, Alexandria, VA. [https://www.serdp-estcp.org/Program-Areas/Environmental-Restoration/Contaminated-Groundwater/Persistent-Contamination/ER-201581/ER-201581 ER-201581][[media:2017-Borden-Post-Remediation_Evaluation_of_EVO_Treatment.pdf| Report.pdf]]</ref>
 ==Introduction==

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-Acids are produced during Enhanced Reductive Dechlorination (ERD) which can cause pH to drop, inhibiting treatment.  Alkaline materials including hydroxides and carbonates are sometimes added to the aquifer during ERD to maintain a pH of greater than 6, improving bioremediation performance.  This article describes a simplified approach and a spreadsheet-based <u>[[Media:Base_Add_Design_Tool_-_August_2018.xlsx|design tool]]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion.  To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.
+Acids are produced during Enhanced Reductive Dechlorination (ERD) which can cause pH to drop, inhibiting treatment.  Alkaline materials including hydroxides and carbonates are sometimes added to the aquifer during ERD to maintain a pH of greater than 6, improving bioremediation performance.  This article describes a simplified approach and a spreadsheet-based <u>[[Media:Base Add Design Tool - Dec 2018.xlsx|design tool]]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion.  To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.
 <div style="float:right;margin:0 0 2em 2em;">__TOC__</div>
@@ Line 15: / Line 15: @@
 '''Key Resource(s):'''
-*Spreadsheet-based <u>[[Media:Base_Add_Design_Tool_-_August_2018.xlsx|Design Tool -- Base Addition for ERD]]</u>
+*Spreadsheet-based <u>[[Media:Base Add Design Tool - Dec 2018.xlsx|Design Tool -- Base Addition for ERD]]</u>
 *[[media:2017-Borden-Post-Remediation_Evaluation_of_EVO_Treatment.pdf| Post-Remediation Evaluation of EVO Treatment: How Can we improve performance]]<ref name = "Borden2017EVO">Borden, R.C., 2017. Post-Remediation Evaluation of EVO Treatment: How Can We Improve Performance. Environmental Security Technology Certification Program, Alexandria, VA. ER-201581[[media:2017-Borden-Post-Remediation_Evaluation_of_EVO_Treatment.pdf| Report.pdf]]</ref>
 ==Introduction==
-Aquifer pH lower than 6 can reduce the efficiency of enhanced reductive dechlorination (ERD) and other in situ remediation processes.  This article describes a simplified approach using a spreadsheet-based <u>[[Media:Base_Add_Design_Tool_-_August_2018.xlsx|design tool]]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion.  This approach requires simplification of several important processes controlling subsurface pH, and is only appropriate for developing initial estimates of the total amount of base required.  In practice, aquifer pH should be monitored during ERD and periodically adjusted to maintain conditions for optimal microbial growth and contaminant degradation. Early in the bioremediation process, reactions will not have gone to completion and less base will be required to maintain the target pH.  To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.
+Aquifer pH lower than 6 can reduce the efficiency of enhanced reductive dechlorination (ERD) and other in situ remediation processes.  This article describes a simplified approach using a spreadsheet-based <u>[[Media:Base Add Design Tool - Dec 2018.xlsx|design tool]]</u> for estimating the total amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion.  This approach requires simplification of several important processes controlling subsurface pH, and is only appropriate for developing initial estimates of the total amount of base required.  In practice, aquifer pH should be monitored during ERD and periodically adjusted to maintain conditions for optimal microbial growth and contaminant degradation. Early in the bioremediation process, reactions will not have gone to completion and less base will be required to maintain the target pH.  To prevent overshoot and excessively high pH, users typically add a fraction of the total base required in several increments spread over time.
 ==Background Aquifer pH==
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 ==Groundwater Acidity==
 Acidity is the amount of base required to neutralize acids present in a water sample. Since various acids can disassociate to different extents, two solutions with the same pH can have different acidities (see [[pH Buffering in Aquifers]]).  In most cases, the acidity of the background groundwater is the sum of acidity from strong mineral acids (phosphoric, nitric, sulfuric, and hydrochloric acids), organic acids, and dissolved carbon dioxide.
-Mineral acidity (also referred to as methyl orange acidity) is determined by titrating a sample with a strong base to pH=3.7 (Standard Method 2310, AWPA 2016).  If the initial pH is greater than 3.7, mineral acidity is zero. The spreadsheet-based <u>[[Media:Base_Add_Design_Tool_-_August_2018.xlsx|design tool]]</u> presented here uses this measurement to account for the base demand of any strong acids already present in the aquifer. Total acidity (commonly referred to as phenolphthalein acidity) should not be used to calculate the amount of base required because the titration endpoint of this parameter is pH 8.3, which will over-estimate the base requirement for optimal ERD.
+Mineral acidity (also referred to as methyl orange acidity) is determined by titrating a sample with a strong base to pH=3.7 (Standard Method 2310, AWPA 2016).  If the initial pH is greater than 3.7, mineral acidity is zero. The spreadsheet-based <u>[[Media:Base Add Design Tool - Dec 2018.xlsx|design tool]]</u> presented here uses this measurement to account for the base demand of any strong acids already present in the aquifer. Total acidity (commonly referred to as phenolphthalein acidity) should not be used to calculate the amount of base required because the titration endpoint of this parameter is pH 8.3, which will over-estimate the base requirement for optimal ERD.
 Under low pH conditions, organic volatile fatty acids (acetic, propionic, etc.) can accumulate and contribute to acidity.  However, in background groundwater, organic acid concentrations are generally low.  In the design tool, acidity from background organic acids is assumed to be negligible.
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 ==Design Tool==
-An MS Excel based <u>[[Media:Base_Add_Design_Tool_-_August_2018.xlsx|design tool]]</u> was developed to aid in estimating the amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion<ref name = "Borden2017EVO"/>.  The general approach and calculations were presented in the previous sections.  The design tool is specifically focused on pH adjustment for ERD and calculation procedures include the following assumptions.
+An MS Excel based <u>[[Media:Base Add Design Tool - Dec 2018.xlsx|design tool]]</u> was developed to aid in estimating the amount of base required to achieve a specified target pH at the end of the treatment period, once all reactions have gone to completion<ref name = "Borden2017EVO"/>.  The general approach and calculations were presented in the previous sections.  The design tool is specifically focused on pH adjustment for ERD and calculation procedures include the following assumptions.
 #The target pH is between 5 and 8.

@@ Line 24: / Line 24: @@
 In humid areas, rainfall combined with carbonic acid produced in the soil leaches out base cations (Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>) gradually acidifying the soil. Figure 1 shows soil pH in the contiguous United States.
-[[File:Borden2w2Fig1.png|thumbnail|center|800 px|Figure 1. Soil pH in the contiguous US ([http://www.bonap.org/2008_Soil/pH20110321.png Soil pH]).  Alkaline soils (pH>7.4) are shown in blue.  Acidic soils (pH<6.1) are shown in brown. ]]
+[[File:Borden2w2Fig1.png|thumbnail|center|800 px|Figure 1. Soil pH in the contiguous US ([http://www.bonap.org/2008_Soil/pH20110321.png Soil pH]).  Alkaline soils (pH>7.4) are shown in blue.  Acidic soils (pH<6.1) are shown in brown.]]
 Groundwater pH is influenced by soil pH, but also by other factors.  As acidic water infiltrates through the soil profile, some of the acidity may be neutralized by dissolution of soil and aquifer minerals.  Silicate minerals including feldspars and micas can hydrolyze over time, consuming acid, and releasing dissolved cations (Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>). However, these weathering reactions are slow, and are often not sufficient to prevent pH declines during ERD.  Figure 2 shows a cumulative frequency distribution of groundwater pH at a site in eastern North Carolina. Average soil pH at this site is approximately 5.  The vadose zone and surficial aquifer at the site are predominantly quartz sand and weathered clays, so weathering processes provide minimal buffering capacity.  90% of the groundwater pH measurements were between 4.5 and 6.0 indicating low soil pH at this site was a reasonable predictor of acidic groundwater.  At other sites containing carbonate minerals or relatively young rocks, mineral dissolution often results in greater buffering.
-[[File:Borden2w2Fig2.png|thumbnail|left|Figure 2. Cumulative frequency distribution of groundwater pH measurements at site in eastern North Carolina where soil pH is approximately 5. ]]
+[[File:Borden2w2Fig2.png|thumbnail|left|Figure 2. Cumulative frequency distribution of groundwater pH measurements at site in eastern North Carolina where soil pH is approximately 5.]]
 ==Groundwater Acidity==
@@ Line 183: / Line 183: @@
 At pH = 6, &alpha; = 0.69 so 0.69 moles of H<sup>+</sup> are consumed per mole of NaHCO<sub>3</sub> and 1.69 moles of H<sup>+</sup> consumed per mole of Na<sub>2</sub>CO<sub>3</sub> (Figure 6).  However at pH = 7, &alpha; = 0.18 so only 0.18 moles of H<sup>+</sup> are consumed per mole of NaHCO<sub>3</sub> and 1.18 moles of H<sup>+</sup> consumed per mole of Na2CO<sub>3</sub>.  As a result, bicarbonates and carbonates are relatively effective at raising the pH to 6.  However, these materials provide less alkalinity per unit mass at pH =7, increasing the amount of material required.
-Ca(OH) <sub>2</sub> and Mg(OH) <sub>2</sub> provide large amounts of OH- per kg.  However, these materials have a low aqueous solubility, making them more difficult to distribute in the subsurface.<ref>Borden, R.C., Lai, Y.S., Overmeyer, J., Yuncu, B. and Allen, J.P., 2016.  In situ pH adjustment with colloidal Mg(OH)<sub>2</sub>.  Environmental Engineer and Scientist: Applied Research and Practice, accepted for publication, Vol 17, pp.28-33.</ref> describe the use of a colloidal form of Mg(OH)<sub>2</sub> with improved transport properties.
+Ca(OH)<sub>2</sub> and Mg(OH)<sub>2</sub> provide large amounts of OH- per kg.  However, these materials have a low aqueous solubility, making them more difficult to distribute in the subsurface.<ref>Borden, R.C., Lai, Y.S., Overmeyer, J., Yuncu, B. and Allen, J.P., 2016.  In situ pH adjustment with colloidal Mg(OH)<sub>2</sub>.  Environmental Engineer and Scientist: Applied Research and Practice, accepted for publication, Vol 17, pp.28-33.</ref> describe the use of a colloidal form of Mg(OH)<sub>2</sub> with improved transport properties.
 ==Design Tool==
@@ Line 210: / Line 210: @@
 The design tool was used to estimate the amount of base required to stimulate ERD in a 435 ft long emulsified vegetable oil (EVO) biobarrier at Operable Unit 2 (OU2) on the former Naval Training Center (NTC) in Orlando, FL<ref name = "Borden2017EVO"/>.  The biobarrier consists of two rows of injection wells spaced approximately 30 ft on center which were treated with a total of 20,224 lb of vegetable oil.  The vertical treatment thickness was 10 ft, influent groundwater pH ~5, and pHBC = 8 meq/kg/pH.  To provide guidance on amounts of base required, the design tool was used to estimate the amount of NaOH, Na<sub>2</sub>CO<sub>3</sub>, NaHCO<sub>3</sub>, or Mg(OH) <sub>2</sub> required to achieve different pH values at the end of the five year treatment period.  Results presented in Figure 7 indicate that the amount of base required is very sensitive to the target pH.  For target pH values < 5.4, little or no base is required.  Increasing the target pH to 6 or 7, requires progressively greater amounts of base because of the soil acidity and carbonic acid released from substrate fermentation.
-To achieve a pH of approximately 7, 10,200 lb of NaOH or 7,400 lb of Mg(OH) <sub>2</sub> are required, which is equivalent to 50% or 37% respectively (by weight) of the vegetable oil injected.  Multiple NaOH injections would be required since a single injection would result in an excessively high initial pH and the base would migrate out of the treatment zone over time with flowing groundwater.  The total mass of Mg(OH) <sub>2</sub> required is less than NaOH, since two moles of OH- are released per mole of Mg(OH) <sub>2.</sub>  However, Mg(OH) <sub>2</sub> has a very low aqueous solubility, so the material must be injected in a colloidal form.  Since the material is a solid and would not migrate downgradient, 100% of the required Mg(OH) <sub>2</sub> could be injected simultaneously with the EVO.  The pH of a pure slurry of Mg(OH)<sub>2</sub> is relatively high (~10.3).  However once injected, CO<sub>3</sub><sup>2-</sup> precipitates on the surface of the Mg(OH) <sub>2</sub> particles forming a MgCO<sub>3</sub> coating that maintains the aquifer pH between 7 and 8<ref>Hiortdahl, K.M. and Borden, R.C., 2013. Enhanced reductive dechlorination of tetrachloroethene dense nonaqueous phase liquid with EVO and Mg (OH)2. Environmental science & technology, 48(1), pp.624-631. [https://doi.org/10.1021/es4042379 doi: 10.1021/es4042379]</ref>.  In general, it is not practical to raise the pH to near 7 with NaHCO<sub>3</sub> due to the small amount of H<sup>+</sup> consumed by this material at near neutral pH.  Na<sub>2</sub>CO<sub>3</sub> could be effective, but would require multiple injections due to the high pH (~11.7) and downgradient migration with groundwater flow.
+To achieve a pH of approximately 7, 10,200 lb of NaOH or 7,400 lb of Mg(OH)<sub>2</sub> are required, which is equivalent to 50% or 37% respectively (by weight) of the vegetable oil injected.  Multiple NaOH injections would be required since a single injection would result in an excessively high initial pH and the base would migrate out of the treatment zone over time with flowing groundwater.  The total mass of Mg(OH)<sub>2</sub> required is less than NaOH, since two moles of OH- are released per mole of Mg(OH)<sub>2</sub>.  However, Mg(OH)<sub>2</sub> has a very low aqueous solubility, so the material must be injected in a colloidal form.  Since the material is a solid and would not migrate downgradient, 100% of the required Mg(OH)<sub>2</sub> could be injected simultaneously with the EVO.  The pH of a pure slurry of Mg(OH)<sub>2</sub> is relatively high (~10.3).  However once injected, CO<sub>3</sub><sup>2-</sup> precipitates on the surface of the Mg(OH)<sub>2</sub> particles forming a MgCO<sub>3</sub> coating that maintains the aquifer pH between 7 and 8<ref>Hiortdahl, K.M. and Borden, R.C., 2013. Enhanced reductive dechlorination of tetrachloroethene dense nonaqueous phase liquid with EVO and Mg (OH)2. Environmental science & technology, 48(1), pp.624-631. [https://doi.org/10.1021/es4042379 doi: 10.1021/es4042379]</ref>.  In general, it is not practical to raise the pH to near 7 with NaHCO<sub>3</sub> due to the small amount of H<sup>+</sup> consumed by this material at near neutral pH.  Na<sub>2</sub>CO<sub>3</sub> could be effective, but would require multiple injections due to the high pH (~11.7) and downgradient migration with groundwater flow.
 ==Summary==

@@ Line 117: / Line 117: @@
 During reductive dechlorination of PCE (C<sub>2</sub>Cl<sub>4</sub>) to TCE (C<sub>2</sub>HCl<sub>3</sub>), a chlorine atom (Cl) is replaced with hydrogen (H<sub>2</sub>) releasing one proton (H<sup>+</sup>) and one chloride (Cl<sup>-</sup>).
-<div class="center"><big>C<sub>2</sub>Cl<sub>4</sub> + H<sub>2</sub> &rArr; C<sub>2</sub>HCl<sub>3</sub> + H<sup>+</sup> + Cl<sup>-</sup>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 5''</div>
+<div class="center"><big>C<sub>2</sub>Cl<sub>4</sub> + H<sub>2</sub> &rArr; C<sub>2</sub>HCl<sub>3</sub> + H<sup>+</sup> + Cl<sup>-</sup>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 5''</div>
 As TCE is further dechlorinated to ''c''DCE, VC and ethene, an additional proton is released for each chlorine removed.  As a result, ERD can release large amounts of acid.  For example, complete reduction of one kilogram of PCE to ethene produces 0.9 kilograms of HCl.
@@ Line 123: / Line 123: @@
 Organic substrates added as electron donors ferment in oxygen-poor environments, releasing H<sub>2</sub> and acetic acid.  Acetic acid rarely accumulates during ERD, because it can be used by various cohorts of organisms for reduction of PCE and TCE, for reduction of background electron acceptors, or for fermentation to methane (CH<sub>4</sub>).  Consumption of acetic acid by all of these processes produces H<sub>2</sub>CO<sub>3</sub>*. The amount of H<sub>2</sub>CO<sub>3</sub>* produced during fermentation of different substrates can be estimated with the following formula:
-<div class="center"><big>C<sub>''&theta;''</sub>H<sub>''&beta;''</sub>O<sub>''&gamma;''</sub> + (3''&theta;'' - ''&gamma;'')H<sub>2</sub>O &rArr; ''&theta;''H<sub>2</sub>CO<sub>3</sub>* + (2''&theta;'' + ''&beta;''/2 - ''&gamma;'')H<sub>2</sub>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 6''</div>
+<div class="center"><big>C<sub>''&theta;''</sub>H<sub>''&beta;''</sub>O<sub>''&gamma;''</sub> + (3''&theta;'' - ''&gamma;'') H<sub>2</sub>O &rArr; ''&theta;'' H<sub>2</sub>CO<sub>3</sub>* + (2''&theta;'' + ''&beta;''/2 - ''&gamma;'') H<sub>2</sub>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 6''</div>
 {|
@@ Line 140: / Line 140: @@
 As shown in Figure 3, CO<sub>3</sub><sup>2-</sup> is only present in significant concentrations at pH > 8.  For pH ranges relevant for ERD (5>pH>8), H<sup>+</sup> release from H<sub>2</sub>CO<sub>3</sub>* can be represented by the following reaction<ref name= "Stumm1996"/>.
-<div class="center"><big>H<sub>2</sub>CO<sub>3</sub>* &rArr; ''&alpha;'' H<sub>2</sub>CO<sub>3</sub>* + (1 - ''&alpha;'') H<sup>+</sup> +  (1 - ''&alpha;'') HCO<sub>3</sub><sup>-</sup> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 7''</div>
+<div class="center"><big>H<sub>2</sub>CO<sub>3</sub>* &rArr; ''&alpha;'' H<sub>2</sub>CO<sub>3</sub>* + (1 - ''&alpha;'') H<sup>+</sup> +  (1 - ''&alpha;'') HCO<sub>3</sub><sup>-</sup> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 7''</div>
 where &alpha; is the fraction of H<sub>2</sub>CO<sub>3</sub>* that does NOT ionize at the target pH with &alpha; = (1+ 10<sup>-6.352</sup> / [H<sup>+</sup>])<sup>-1</sup>.  Figure 6 shows the variation in &alpha; as a function of pH.  At pH 6, &alpha; = 0.69, so 0.31 moles of H<sup>+</sup> are released for every mole of CO<sub>2</sub> produced.  However at pH 7, &alpha; = 0.18, so 0.82 moles of H<sup>+</sup> are released for every mole of CO<sub>2</sub> produced.
@@ Line 172: / Line 172: @@
 | Magnesium<br/>Hydroxide ||Mg(OH)<sub>2</sub> || 58.3 || 2 || 34.3 || <0.01 || ~10.3
 |}
-<br/>
 NaOH and KOH provide a large number of OH- equivalents (eq) per kg and are very soluble so only small volumes of base are required to raise the aquifer pH.  However, concentrated solutions of NaOH and KOH have pH > 14 which is inhibitory to bacteria, would expose workers to safety hazards, and can partially dissolve aluminosilicates.
 As described above, solid calcium carbonate (CaCO<sub>3</sub>) is generally not effective in raising pH during ERD because of its low aqueous solubility.  Na<sub>2</sub>CO<sub>3</sub> and NaHCO<sub>3</sub> are much more soluble and can be effective in raising aquifer pH.  The amount of H<sup>+</sup> consumed per mole varies as a function of pH between pH 5 and 8<ref name= "Stumm1996"/>.  For closed conditions (below water table where CO<sub>2</sub> cannot degas) and pH < 8, NaHCO<sub>3</sub> and Na<sub>2</sub>CO<sub>3</sub> disassociate to H<sub>2</sub>CO<sub>3</sub>* and HCO<sub>3</sub><sup>-</sup>, consuming H<sup>+</sup> by the following reactions.
-<div class="center"><big>NaHCO<sub>3</sub> + ''&alpha;'' H<sup>+</sup> &rArr; Na<sup>+</sup> + ''&alpha; '' H<sub>2</sub>CO<sub>3</sub>* + (1 – ''&alpha;'') HCO<sub>3</sub><sup>-</sup> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 8''</div>
+<div class="center"><big>NaHCO<sub>3</sub> + ''&alpha;'' H<sup>+</sup> &rArr; Na<sup>+</sup> + ''&alpha; '' H<sub>2</sub>CO<sub>3</sub>* + (1 – ''&alpha;'') HCO<sub>3</sub><sup>-</sup> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 8''</div>
-<div class="center"><big>Na<sub>2</sub>CO<sub>3</sub> + (1 + ''&alpha;'') H<sup>+</sup> &rArr; 2 Na<sup>+</sup> + ''&alpha;'' H<sub>2</sub>CO<sub>3</sub>* + (1 – ''&alpha;'') HCO<sub>3</sub><sup>-</sup> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 9''</div>
+<div class="center"><big>Na<sub>2</sub>CO<sub>3</sub> + (1 + ''&alpha;'') H<sup>+</sup> &rArr; 2 Na<sup>+</sup> + ''&alpha;'' H<sub>2</sub>CO<sub>3</sub>* + (1 – ''&alpha;'') HCO<sub>3</sub><sup>-</sup> &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; </big> ''Reaction 9''</div>
 At pH = 6, &alpha; = 0.69 so 0.69 moles of H<sup>+</sup> are consumed per mole of NaHCO<sub>3</sub> and 1.69 moles of H<sup>+</sup> consumed per mole of Na<sub>2</sub>CO<sub>3</sub> (Figure 6).  However at pH = 7, &alpha; = 0.18 so only 0.18 moles of H<sup>+</sup> are consumed per mole of NaHCO<sub>3</sub> and 1.18 moles of H<sup>+</sup> consumed per mole of Na2CO<sub>3</sub>.  As a result, bicarbonates and carbonates are relatively effective at raising the pH to 6.  However, these materials provide less alkalinity per unit mass at pH =7, increasing the amount of material required.
-Ca(OH) <sub>2</sub> and Mg(OH) <sub>2</sub> provide large amounts of OH- per kg.  However, these materials have a low aqueous solubility, making them more difficult to distribute in the subsurface.<ref>Borden, R.C., Lai, Y.S., Overmeyer, J., Yuncu, B. and Allen, J.P., 2016.  In situ pH adjustment with colloidal Mg(OH)<sub>2</sub>.  Environmental Engineer and Scientist: Applied Research and Practice, accepted for publication, Vol 17, pp.28-33.</ref> describe the use of a colloidal form of Mg(OH) <sub>2</sub> with improved transport properties.
+Ca(OH) <sub>2</sub> and Mg(OH) <sub>2</sub> provide large amounts of OH- per kg.  However, these materials have a low aqueous solubility, making them more difficult to distribute in the subsurface.<ref>Borden, R.C., Lai, Y.S., Overmeyer, J., Yuncu, B. and Allen, J.P., 2016.  In situ pH adjustment with colloidal Mg(OH)<sub>2</sub>.  Environmental Engineer and Scientist: Applied Research and Practice, accepted for publication, Vol 17, pp.28-33.</ref> describe the use of a colloidal form of Mg(OH)<sub>2</sub> with improved transport properties.
 ==Design Tool==