Abstract:

Low-molecular-weight organic acids (LMWOAs) are important components of dissolved organic matter (DOM) in soil. They are widely distributed and play an important role in controlling the geochemical behavior of heavy metal(loid)s in soil environments. In this study, As-contaminated paddy soil was sampled and biochar was produced from Paulownia fortune. The effects of oxalic acid (OA) and tartaric acid (TA), two common LMWOAs, on As desorption from the soil were investigated. The role of biochar in altering As desorption was assessed and the mechanisms involved were investigated. The results showed that both OA and TA decreased soil pH in a concentration-dependent manner; OA had a stronger effect than TA, and soil pH decreased to 1.89 and 2.62 for OA and TA at 40 mmol∙L^-1. Biochar addition alleviated the pH reduction, reducing the pH to 2.34 and 2.72, indicating that biochar enhanced soil pH values of 0.45 and 0.10 OA and TA, respectively. OA and TA had significant effects on As desorption from the soil. The arsenic desorption mass increased with the OA and TA concentrations, and OA induced a higher As desorption mass than TA. The Addition of biochar increased the AS desorption mass by 15%-98% and 21%-509% for OA and TA, respectively. The primary possible mechanisms by which TA and OA alter As desorption from the soil are pH reduction and complexation with As via carboxyl groups. The addition of BC increased As desorption, mainly owing to the introduction of DOM and protonation caused by biochar. This study deepened our understanding of the effects of LMWOAs on As geochemistry in soil environments and revealed the potential risks of biochar use as an amendment in As-contaminated soils.

Key words: tartaric acid, oxalic acid, biochar, arsenic, desorption behavior

摘要：

低分子量有机酸（Low molecular weight organic acids，LMWOAs），如草酸（Oxalic Acid，OA）和酒石酸（Tartaric Acid，TA），是土壤水溶性有机质（DOM）的重要组成部分，对土壤（类）重金属的吸附和解吸行为具有明显的影响。以As污染超标土壤为研究对象，利用泡桐制备的生物炭（BC），通过批解吸试验研究了TA和OA对土壤中As解吸行为的影响，分析了BC对OA和TA影响土壤As解吸行为的作用，探讨了可能的作用机理。结果表明，OA和TA均能降低土壤pH，且其浓度越高pH值越小；OA对土壤pH的作用强于TA，在40 mmol∙L⁻¹时pH分别降为1.89和2.62。BC的添加未改变土壤pH降低的趋势，但减小了pH降低的幅度，OA和TA在40 mmol∙L⁻¹时使pH分别降为2.34和2.72，BC使土壤pH分别提升了0.45和0.10。OA和TA明显影响土壤As的解吸行为，As的解吸量随OA或TA浓度的增加而先降低后增加；OA对土壤As的解吸量明显高于TA。BC的添加未改变土壤As的解吸趋势，但使OA和TA对土壤As的解吸量分别增加了15%-98%和21%-509%。OA和TA可能是通过降低pH和羧基结合作用而影响土壤As的解吸行为，BC的添加可能提升DOM含量和土壤表面质子化反应而促进As的解吸。结合Langmuir和Freundlich模型分析，OA和TA对土壤As的解吸主要以其与As化学结合作用为主。该研究成果有助于加深对LMWOAs对土壤As解吸行为影响的认识，明晰了生物炭修复As污染土壤存在的风险，对土壤污染防控具有重要的参考价值。

关键词: 酒石酸, 草酸, 生物炭, 砷, 解吸行为