氯酸盐屏蔽废水COD测定的消除策略及反应机制

Elimination strategy and reaction mechanism for the interference of chlorate on COD

向废水中投加氯酸盐,即所谓的“COD(化学需氧量)去除剂”,可降低COD检测值,呈现出水符合COD排放标准的假象.从本质上而言,氯酸盐的添加并未实际去除COD,而仅仅是作为氧化剂减少了在高温消解过程中重铬酸钾的消耗量,导致测定的COD数值偏小,因此氯酸盐只是起到了掩蔽作用.为消除氯酸盐对COD的掩蔽作用,提出了一种以亚硫酸盐为还原剂的预处理工艺:根据氯酸盐含量投加相应的亚硫酸氢钠进行去除,进而测定出真实的COD.结果表明,当[SO_(3)^(2-)][ClO_(3)^(-)]摩尔比为5-6、温度为60℃时,经过1h的反应可以有效的消除氯酸盐对COD测定的影响,此时SO_(3)^(-)和ClO_(3)^(-)同步耗尽.基于ESR(电子自旋共振)分析,可知氯酸盐与亚硫酸盐首先通过单电子转移反应生成SO_(3)^(-),生成的SO_(3)与溶解氧进一步发生反应生成SO_(4)^(-),而CO_(3)^(-)的还原中间体(ClO_(2)、ClO_(2),和CIO^(-))可与亚硫酸根继续通过氧转移反应而被还原,最终产物为Cl^(-);通过反应前后的三维荧光光谱图发现生成的自由基虽然可以氧化部分有机物,但是并未发生矿化反应,因此经处理后所测定的COD值即为真实值。总而言之,本研究所提出的亚硫酸盐消除氯酸盐对COD测定干扰的方法切实可行,为完善COD测定方法及理论提供参考.

Deliberate addition of mildly oxidative chlorate(ClO_(3)^(-)),so-called“COD(chemical oxygen demand)degrader”,into wastewater induces the false COD reduction,which would bring about false appearance of effluents meeting the COD discharge standards.In essence,chlorate doesn’t really remove COD but just masks the COD determination process,where it acts as an alternative oxidant to dichromate and therefore the consumption of dichromate is reduced.Herein,a pretreatment process using sulfite as reducing agent was proposed to reduce chlorate and therefore eliminate the masking effect of chlorate on COD determination.It was demonstrated that the COD value could be restored well after 1 h reduction of ClO_(3)^(-)by sulfite at 60℃ with the[sulfite]/[chlorate]molar ratio value of 5-6,accompanying the synchronously complete depletion of these two reactants.Based on ESR(electron spin-resonance spectroscopy)analysis,the reaction between CiO;and sulfite mainly proceeded via one-electron transfer process with the generation of SO_(3),the subsequent formation of SO_(4)^(-) derived from the oxidation of SO;by the dissolved O_(2).The ClO_(3)^(-)reduction intermediates,such as ClO_(2),ClO_(2)^(-),and ClO^(-),could be further reduced by sulfite via oxygen transfer process with CI as the final product.Although the three-dimensional fluorescence spectra indicating that the generated free radicals could oxidize part of the organic matter,no mineralization reaction occurred,so the COD value measured after the treatment is the true value.In general,this study pioneered an effective pretreatment method using sulfite as reducing agent to eliminate the interference of chlorate on COD determination and provide reference for improving COD determination method and theory.