Theme: 01. Remediation Technology Innovations

Topic Area: 01d. Combined Remedies and Treatment Train Technologies

Formatted Title:

Bench Testing for Transforming Methyl Methacrylate via Base-Catalyzed Hydrolysis

Background/Objectives:

A large volume release of a bulk load of methyl methacrylate (MMA) occurred in a rural area, impacting productive agricultural soils and groundwater. Excavation of impacted soil was completed to the extent practicable with significant MMA detected in excavation sidewalls and beneath existing infrastructure where further excavation was not desirable due to infrastructure stability concerns. MMA hydrolyses produces methanol (MeOH) as a significant daughter product. MMA concentrations in soil already exceeded land disposal restrictions and gradually increasing MeOH concentrations observed in situ at the spill site were also considered a hazardous constituent both from a disposal perspective and as a soil migration to groundwater risk. Hydrolysis half-life of MMA is estimated at greater than 4 years; and volatilization is anticipated to be a more important fate process based on MMA’s Henry’s Law constant.

Approach/Activities:

MMA is volatile and very soluble in water with high mobility in groundwater. It should tend to sorb to soils based on Koc values. However, there are few studies of methods for decomposition or remediation of MMA. The compound is an ester, and esters are created via reaction of a carboxylic acid with an alcohol. Esters can be converted back to the carboxylic acid and alcohol parent via hydrolysis. A significant consideration before promoting the process of MMA hydrolysis is addressing the production of MeOH and its subsequent accumulation. Under acidic conditions, the accumulated methanol and MMA reach equilibrium, requiring the removal of methanol to drive the reaction forward. However, under basic conditions, carboxylic acid formed during the reaction is further deprotonated by hydroxide ions resulting in carboxylate ion formation. Conducting reactions under basic conditions (base-catalyzed esters hydrolysis) proceeds via an addition-elimination sequence via hydroxide ion, with subsequent deprotonation (or ionization) of carboxylic acids into their salt (carboxylate ion and cation) and water. This reaction mechanism is also seen in something familiar: soap preparation (saponification) where fats and oils (esters) are transformed by alkaline hydrolysis into “soap”. A bench study was designed and conducted to examine the efficacy of a conceptualized approach that would attempt to hydrolyze the MMA under basic conditions while oxidizing the MeOH as it formed to demonstrate concurrent transformation of MMA into carboxylate ion with destruction of the daughter product of concern (MeOH).

Results/Lessons Learned:

The experimental design incorporated practical considerations for potential on-site site implementation. Two kinds of alkaline reagents are both readily available and relatively easy to apply in situ: sodium hydroxide (NaOH) or hydrated lime (Ca(OH)₂). Rapid destruction of MeOH (and possibly MMA) is achievable via oxidation using sodium persulfate (SP) or Modified Fenton’s Reagent (MFR). Experiments examined the base buffering capacity of site soils, the oxidant demand for site soils, and the stability of MFR in matrix materials (site soils and groundwater). The efficacy of alkaline hydrolysis was examined using NaOH (more readily soluble in water than, while having simple molar hydroxide equivalence to, Ca(OH)₂) in an initial screening test to help identify optimal pH for MMA alkaline hydrolysis. Subsequent tests evaluated the effectiveness of MFR, calcium hydroxide activated sodium persulfate (CaSP) and sodium hydroxide activated sodium persulfate (NaSP) in treating MMA and MeOH, at pH determined by the preceding alkaline hydrolysis test.

Buffer capacity testing indicated hydroxide buffering capacity to achieve pH 10 target was approximately 1.2 g/kg. Evaluation of alkaline hydrolysis occurred at pH 8, 10, and 12. Alkaline hydrolysis promoted effective MMA concentration reduction by transforming MMA into its daughter product MeOH. Alkaline conditions of pH 12 resulted in greatest MMA mass reduction (99%) while pH 10 produced greatest relative increase in MeOH concentrations. Lower pH tests appeared to result in MMA desorption from soils and the generation of increasing MMA and MeOH concentrations. Based on results, pH 11 was selected for oxidation testing under alkaline conditions. Reactions were conducted using two different doses of oxidant for CaSP, NaSP, and MFR treatment conditions. All conditions effectively reduced MMA concentrations in soil and groundwater. However, the 30 g/kg dose of MFR was most efficient for oxidation of resultant MeOH. A challenge is that the mass of MeOH generated exceeded available oxidant at the doses tested, suggesting a series of MFR re-applications (multiple in-situ injections) would likely be needed for complete MeOH treatment. As the basic conditions established by either NaOH or Ca(OH)₂ were equally effective for base hydrolysis of MMA, the recommended approach was backfilling hydrated lime or liquid NaOH amendments to clean soil and mixing within the existing excavation. MFR could be added during the mixing process and during subsequent in situ injections.

Yan Chin

Affiliation

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Bench Testing for Transforming Methyl Methacrylate via Base-Catalyzed Hydrolysis