形态分析中铬的价态变化及其有效态预测

Valence variation of chromium in the chemical speciation analysis and prediction models for the available chromium in slag with chromium

为了判定连续浸提法的准确度和重金属污染评价的可信性,选择铬元素为目标重金属,以Tessier连续浸提法中的第三步和第四步使用的浸提剂盐酸羟胺(NH_2OH·HCl)和双氧水(H_2O_2)分别作为还原剂和氧化剂,研究铬的形态分析中三价铬(Cr3+)和六价铬(Cr^(6+))相互转化的可能性与可进行程度。此外,以铬渣为原料,考察pH值、温度和振荡时间对铬渣中铬有效态浸出量的影响。结果表明,NH_2OH·HCl在pH值为2~6时均可将Cr^(6+)还原为Cr3+,还原度达90%以上,由此会低估Cr^(6+)的潜在危害。pH值小于7时,Cr3+仅作为催化剂加快H_2O_2的分解,Cr3+含量几乎不变。将pH值、温度和振荡时间对铬离子的浸出影响关联为单因素和双因素数学模型,可预测不同条件下铬渣中Cr^(6+)和总铬(TCr)的浸出量。

This paper intends to make an exploration of the possibility and the extent of mutual conversion between the trivalent chromium（ Cr^（3＋）） and the hexavalent chromium（ Cr6＋） by using the leaching agent hydroxylamine hydrochloride（ NH2OH·HCl）and the hydrogen peroxide（ H2O2） as the reducing and oxidizing agents in the third and the fourth steps in the Tessier sequential extraction,respectively,for the existence of some heavy metal in the various valence states may lead to the variance of the valence with the leaching agents being reducible or oxidize-liabilities. As is known,heavy metals of various valences tend to result in different biological availability and mobility,which may in turn influence the evaluation of heavy-metal contamination. What is more,the complicated steps in the sequential extraction for the chemical speciation analysis of the heavy metal contaminants are likely to lead to a certain amount of errors. It is just due to the complicated reasons that have pushed us to establish a mathematic model for the prediction of the leaching content of the available chromium in the said slag by investigating the influential factors,including the pH value,temperature and oscillation time length.The results of our investigation and analysis show that there exists the higher oxidation-reduction potential and the larger reaction equilibrium constant of the reactions between chromium and the leaching agents,which may therefore result in the valence variation in correspondence with the experimental findings. Moreover,Cr6＋can also be made converted to Cr3 ＋by NH2 OH · HCl in case the pH value can be kept between 2 to 6,which may help to raise the reduction degree by over 90%. Therefore,it can be said that the potential hazard of Cr6＋has been underestimated. However,when the pH value is supposed to be less than 7,it would be possible for Cr3 ＋to accelerate the decomposition of H2 O2 as a catalyst whereas the content of Cr3 ＋itself remains almost unchanged. In addition,it can be concluded that the influence of pH value,temperature and oscillation time on the leaching of chromium in the chromium slag is related with the single-factor and double-factor models at a higher correlated coefficient in a significant degree. Hence,the above models can be expectantly used to evaluate the leaching amount of Cr6＋and the total chromium（ TCr） in the chromium slag under different conditions.