土壤铵氮在热活化过硫酸盐氧化过程中的转化

采用来自江苏和河北, 具有不同土壤有机质含量和NH₄⁺浓度的土壤样本, 系统地研究了NH₄⁺在热活化过硫酸盐(PS)氧化过程中的转化和归趋, 考察了反应时间、PS浓度和外加NH₄⁺对硝基副产物生成的影响.结果表明, 土壤中的NH₄⁺能够转化成3-硝基酚、4-硝基酚、2-羟基-5-硝基苯甲酸、4-羟基-3-硝基苯甲酸、2, 4-二硝基酚等副产物, 它们的生成量随着反应的进行先增加后降低.增大PS浓度可促进硝基副产物的生成.当PS浓度为30mmol/kg, 反应12h后一硝基酚和一硝基羟基苯甲酸的生成量达到最大.然而随着PS浓度进一步增大, 硝基副产物发生降解.硫酸根自由基(SO₄·^-)在硝化过程中起到了关键作用, 它能将NH₄⁺氧化生成氨基自由基(·NH₂), 随后经过一系列自由基链式反应生成二氧化氮自由基(NO₂·).同时, SO₄·^-进攻土壤有机质中的酚结构单元, 使其氧化生成苯氧自由基, 苯氧自由基进一步与NO₂·结合生成硝基副产物.天然有机质(NOM)在环境中无处不在, NH₄⁺在环境中也普遍存在, PS用于土壤和地下水污染修复时生成硝基副产物很可能是一个普遍现象.

Transformation of soil ammonium nitrogen in the process of thermally activated persulfate oxidation

In order to explore the transformation and fate of soil NH₄⁺ in the thermally activated PS oxidation process, this study used soil samples collected from Jiangsu and Hebei provinces with different soil organic matter content and NH₄⁺ concentration to conduct experiments, and systematically investigated effects of persulfate (PS) concentration, the addition of NH₄⁺, and reaction time on the formation of nitro by-products. Results show that soil NH₄⁺ could be transformed to nitrated byproducts, including 3-nitrophenol, 4-nitrophenol, 2-hydroxy-5-nitrobenzoic acid, 4-hydroxy-3-nitrobenzoic acid, 2, 4-dinitrophenol, etc. The formation of nitro by-products increased first and then decreased with reaction time. An increased in PS dose would promote the formation of nitro by-products, and the yields of mono-nitrophenols and hydroxy-mono-nitrobenzoic acids reached the maximum after 12h reaction at 30mmol/kg PS dose. However, nitrated byproducts were degraded at higher PS dose. Note that sulfate radicals (SO₄·^-) played a key role in the nitration process by oxidizing NH₄⁺ to form aminyl radicals (·NH₂), and then underwent a series of free radical chain reactions to form nitrogen dioxide radicals (NO₂·). Besides, phenol moieties in soil organic matter served as the main reactive sites for SO₄·^- attack, leading to the formation of phenoxy radicals, which further combined with NO₂· to form nitro by-products. NOM is everywhere and NH₄⁺ is ubiquitous in the environment. Thus, the formation of nitro by-products will be widespread when PS is applied for contaminated soil and groundwater remediation, which should be taken into consideration when evaluating the feasibility of this technology. This study reveals that the presence of soil NH₄⁺ in activated PS oxidation processes could induce the nitration of NOM and the formation of nitrophenolic by-products.