环境水质学国家重点实验室知识库

      水源地的原水常含有微量的亚硒酸盐[SeO32−，Se(IV)]和碘离子（I¯）等无机阴离子，在氧化/消毒过程中易与天然有机物（Natural organic matter，NOM）反应产生相应的消毒副产物（Disinfection by-products，DBPs），如硒酸盐[SeO42−，Se(VI)]和碘代DBPs（Iodinated disinfection by-products，I-DBPs）。近来，DBPs的生成问题已成为饮用水处理领域的研究热点之一，因此，探索这些无机阴离子在饮用水处理及输配过程中的转化动力学与机制，可为全过程评估DBPs 的生成及健康风险提供理论依据和技术支持。
      天然水体中硒的主要存在形态为Se(IV)和Se(VI)，其中Se(IV)具有高的生物可利用性且毒性为Se(VI)的5-10 倍。我们研究了Se(IV)与常用及次生氧化/消毒剂（HOCl、HOBr、NH2Cl、O3、KMnO4 和H2O2）的反应动力学与机制，并建立了相应的动力学模型。结果表明，在pH 7.0 及氧化剂过量条件下，上述反应均符合准一级动力学，相应的表观二级反应速率常数（k"app）在(1.5 ± 0.1) × 10 4(5.8 ± 0.3) × 103 M1 s1 范围之间；其中O3 与Se(IV)反应较快，而NH2Cl 和H2O2与Se(IV)的反应相对较慢。在氯化过程中，随着Br¯的浓度从65 增加到215 μg L1，Se(VI)生成率由45%增加到约90%。虽然水中共存的Br¯能促进氯化过程中Se(IV)的转化，但并不影响O3 对Se(IV)的转化。不同氧化剂与Se(IV)的反应速率顺序为：•OH > HOBr > O3 > HOCl > KMnO4 > NH2Cl > H2O2，与量子化学计算的顺序一致。
     由于输配管网中通常存在含铅管垢腐蚀物（主要成分为PbO2），我们研究了不同pH 值、I¯和PbO2 浓度等因素对PbO2 氧化I¯的动力学以及相应I-DBPs 生成的影响。结果表明，PbO2 氧化I¯为多级反应，其氧化速率方程可表示为：R = 6.92× 109 × [SAPbO2]0.67 × [I¯]2.19 × [H+]0.59；PbO2 氧化I¯的速率随着pH 的增加而降低；I2 为PbO2 氧化I¯生成活性碘的主要组分，其继续氧化生成IO3¯。基于PbO2 对中间产物I2 和HOI 转化的影响，建立了PbO2/I¯氧化动力学模型，可较好模拟I¯、I2 和IO3¯浓度的变化规律。同时发现，不同类型的含铅氧化物会影响I-DBPs 的生成，其次序为：Pb3O4 > PbO2 > Pb(CO3)2(OH)2 > Pb2O3；生成的I-DBPs 以碘仿和一碘乙酸为主，而且I-DBPs 的生成量随着pH 的增加而降低。提出了NOM 共存条件下，PbO2 与I¯的反应机制：（1）PbO2 对I¯的快速吸附和缓慢氧化（产物为I2）；（2）PbO2 将部分吸附的I2 进一步氧化成IO3¯，同时释放出Pb2+；（3）在PbO2 催化下，其余吸附的I2 与NOM 反应生成I-DBPs；（4）I2 进一步缓慢水解生成HOI，后者与NOM 反应生成I-DBPs；（5）PbO2 将NOM 氧化成NOMox，后者与I¯/I2 继续反应生成I-DBPs。

     Trace inorganic aions, such as selenite [SeO32, Se(IV)] and iodide (I¯) ions, are naturally present in drinking water sources, which can react with natural organic matte to generate disinfection by-products (DBPs) such as selenate [SeO4 2, Se(VI)] and iodinated DBPs (I-DBPs) during water disinfection/oxidation processes. Recently, the formation of DBPs has become one of the research hotspots in the field of drinking water treatment. Consequently, exploring the transformation kinetics and mechanisms of trace inorganic aions during water treatment and distribution will provide theoretical basis and technical support for evaluating the formation of DBPs and their health risks.

     In natural waters, dissolved inorganic selenium (Se) occurs primarily as a combination of Se(IV) and Se(VI). Se(IV) has a higher bioavailability and is approximately 5–10 times more toxic than Se(VI). We investigated the reaction kinetics and mechanisms of Se(IV) with typical water oxidants/disinfectants [i.e.,hypochlorous acid (HOCl), hypobromous acid (HOBr), monochloramine (NH2Cl), ozone (O3), potassium permanganate (KMnO4), and hydrogen peroxide (H2O2)], and developed the relevant kinetic models as well. Results indicate that all the reactions followed the pseudo-first-order kinetics with excessive oxidant at pH 7.0, and the
corresponding apparent second-order rate constants (k"app) varied from 1.5 (± 0.1) ×104 to 5.8 (± 0.3) × 103 M1 s1. The reaction between O3 and Se(IV) proceeded fast, but that between NH2Cl (or H2O2) and Se(IV) was relatively slow. As Br¯ concentration increased from 65 to 215 μg L1, the conversion ratio of Se(IV) to Se(VI) increased from 45% to nearly 90% during chlorination. Although the presence of Br¯ promoted the conversion of Se(IV) during chlorination, no promotion effect was observed during ozonation. The reaction rates of various oxidants with Se(IV) in aqueous solution decreased in the following order: HOBr > O3 > HOCl > KMnO4 >NH2Cl > H2O2, a sequence similar to that predicted by quantum chemical calculations.

     Because water distribution pipes usually have lead-containing corrosion products (main component PbO2), we investigated the effects of pH, I¯ and PbO2 concentrations on the oxidation kinetics of I¯ by PbO2 and the I-DBPs formation. Results indicated that the oxidation of I¯ by PbO2 was a multi-order reaction, whose rate could be expressed by the following equation: R = 6.92 × 109 × [Surface area of PbO2]0.67 × [I¯]2.19 × [H+]0.59. The reaction rate decreased with increaseing pH. I2 was
the major intermediate formed during the oxidation of I¯ by PbO2, which could be further oxidized to IO3-. Based on the effect of PbO2 on the transformation of the intermediates (I2 and HOI), a kinetic model was developed that could well simulate the evolution of I¯, I2 and IO3
¯ in the PbO2/I¯ reaction system. Furthermore, the various forms of lead oxides could affect the I-DBPs formation with a descending order as follows: Pb3O4 > PbO2 > Pb(CO3)2(OH)2 > PbCO3. Iodoform and
monoiodoacetic acid were identifided as the main I-DBPs, and I-DBPs formation decreased with increasing pH. In the presence of natual organic matter (NOM), the reaction mechansism between PbO2 and I¯ was proposed: (1) rapid complexation of I¯ on PbO2 surface and subsequently slow I2 formation; (2) partial oxidation of the surface-bound I2 to IO3
¯ and simultaneous release of Pb2+; (3) PbO2-catalyzed formation of I-DBPs from the reaction of other surface-bound I2 with NOM; (4) I2 is
slowly hydrolyzed to form HOI, which reacts with NOM to form I-DBPs; and (5) direct oxiditon of NOM (NOMox) by PbO2 and subsequent I-DBPs formation from the reactions of NOMox with I¯/I2.