Effect of acid or alkali spills on physicochemical properties and pH buffering capacity of soil and its implication on soil vulnerability

Effect of acid or alkali spills on physicochemical properties and pH buffering capacity of soil and its implication on soil vulnerability Inhyeong Jeon Civil and Environmental Engineering The Graduate School Seoul National University Acid or alkali spills occurs annually and accounts for around 50% of the total chemical accident in South Korea. Physicochemical properties of soils including organic matter content, cation exchange capacity (CEC), base saturation, and exchangeable cations changed as a result of acid or alkali spills. After these spills, as a part of the remediation, a neutralization, pouring neutralizing agents onto the acidor alkali-spilled soil, has been performed, but it could not recover irreversible alteration of soil properties, such as dissolution of organic matter and structural modification of clay minerals. Due to the changes in soil properties, soil functions, particularly pH buffering capacity, might be altered despite neutralization. In addition, since organic matter content decreased after acid or alkali spills, the clay minerals would play a significant role in pH buffering capacity of soils. Knowledge of the effect of acid or alkali spills on soils is essential in order to manage acidor alkali-spilled sites properly; Thus, we investigated the effect of acid or alkali spills on soils and identified characteristics of soils which exhibit vulnerability to the spills. In addition, changes in surface properties of clay minerals as a result of the spills and its effect on pH buffering capacity of clay minerals were investigated. Twenty field soils were collected and reacted with HCl and NaOH, respectively (i.e. one gram of soil received 30 mmol of HCl and NaOH, respectively), to simulate acid or alkali spills. The soils were neutralized to pH 6-8 and freeze-dried. Organic matter and clay content, and cation exchange capacity (CEC) of three different soils (untreated, acid-spilled, and alkali-spilled) were measured and pH buffering capacity, which is an indicator of soil function, was derived from titrations. In addition, statistical analyses including clustering analysis were conducted to identify characteristics of vulnerable soils. Acid or alkali spills significantly decreased organic matter from 3.25% to 1.55 and 1.00%, respectively, and decreased CEC from 13.29 cmol kg to 9.14 and 6.79 cmol kg, respectively. No change in clay content was detected. The pH buffering capacity decreased from 32.96 mmol kg pH unit to 20.26 and 18.48 mmol kg pH unit, respectively. The pH buffering capacity of untreated soils was strongly predicted (79%) by linear combination of organic matter and clay content, while that of acidor alkali-spilled soils was well predicted by only clay content (61 and 80%, respectively) due to decrease in organic matter. Of the three clustered soil groups, the group of which pH buffering capacity is greater than 50 mmol kg pH unit was the most vulnerable to acid or alkali spills, and pH buffering capacity was decreased by 65%. This might be due to a greater organic matter content in this group, which increases vulnerability to the spills. To investigate the effect of acid or alkali spills on clay minerals, kaolinite and montmorillonite were selected as 1:1 and 2:1 clay minerals, respectively. Because surface properties including site density of surface functional groups and their surface acidity constants (i.e. pKa1 and pKa2) determined pH buffering capacity of clay minerals, the effect of acid or alkali spills on surface properties was investigated. Surface complexation modeling with potentiometric titrations and Fourier-transform infrared spectroscopy showed that acid or alkali spills did not significantly change the surface properties of kaolinite. In montmorillonite, however, acid spills decreased the basal site density from 832 to 737 mmol kg by dissolving substituted octahedral cations and decreased pKa2 from 7.32 to 5.42 by dissolving SiOH. In response to alkali spills, the basal site density increased to 925 mmol kg through the isomorphic substitution, and the edge site density increased from 84.8 to 253.2 mmol kg due to AlOH and SiOH formation; thus, pKa2 decreased to 6.78. The pH buffering capacity of acidor alkali-spilled kaolinite at pH 6 did not significantly change, while that of acidor alkali-spilled montmorillonite increased from 30.33 to 35.89 and 56.03 mmol kg pH unit, respectively. Our results indicate that these spills greatly altered the surface properties of montmorillonite, but unexpectedly, increased the pH buffering capacity of montmorillonite. This study clearly shows that acid or alkali spills did not have an adverse effect on pH buffering capacity of clay minerals, and that soil vulnerability to acid or alkali spills can be predicted based on soil properties, and it is useful to classify vulnerable soils in the areas with a high probability of spills, and to manage these spilled areas on a sitespecific basis. Keyword: Acid or alkali spill, soil property, pH buffering capacity, clay mineral, surface complexation modeling Student Number: 2017-20226