Speciation Prediction Of Chromium And Stability Evaluation Of Chromium Adsorbed On Activated Carbon

The risk assessment of heavy metal pollution plays a key role in the pollution control.The sequential extraction for the speciation of heavy metal has been used for the evaluation of heavy metal contamination,but the analytic results are not accurate due to the multiple steps,complicated operations and various types of reagents.In this paper,the effect of the leaching agent on the valence of chromium during sequential extraction was studied.Also,the mathematic models were established for the prediction of leaching content of the available chromium in chromium slag to simplify the procedures and improve the effectiveness of the assessment.In addition,we explored the methods of evaluating the stability of heavy metals adsorbed by activated carbon based on our previous work so as to provide the support for the development of stabilization of heavy metals with activated carbon.In this paper,leaching agent hydroxylamine hydrochloride（NH₂OH·HCl）and hydrogen peroxide（H₂O₂）used at the third step and fourth step in Tessier sequential extraction were employed respectively as reducing and oxidizing agents to explore the possibility and extent of interconvert between trivalent chromium（Cr（Ⅲ））and hexavalent chromium（Cr（VI））.The results showed that Cr（VI）can be converted to Cr（Ⅲ）by NH₂OH·HCl when p H value was kept between 2 to 13.Thus,the potential hazard of Cr（VI）would be underestimated since the toxicity of Cr（VI）is more than100 times of that of Cr（III）.The influences of p H value,temperature and oscillation time on the leaching of total chromium(T_Cr)and Cr（VI）in chromium slag were related by single-factor and double-factor models with higher correlation coefficient and significance level of R²=0.66～0.91,P≤0.05.These models were expected to evaluate the leaching amount of Cr（VI）and T_Cr in chromium slag under different conditions.The modified activated carbons（OAC,NAC and SAC）were prepared respectively by the oxidation with nitric acid,the treatment with ammonia and the immersion with sodium sulfide in the presence of commercial activated carbon（AC）as raw materials.The properties of the activated carbons were characterized by N₂adsorption,SEM,TEM,Boehm titration,FTIR,p H_pzc and XPS.The results indicated that the specific surface area of OAC was increased by 18.6%,with the increasing carboxyl,carbonyl and hydroxyl oxygen containing functional groups;the specific surface area of NAC was increased by 17.3%with the increasing N-containing groups as the pyrrole,pyridine and amine;the specific surface area of SAC was decreased by 18.6%with introducing thiophene,sulfoxide sulfur functional groups.The adsorption stability of Cr（VI）on the activated carbons was evaluated by thermogravimetric analysis,X-ray photoelectron spectroscopy binding energy analysis and the specific adsorption.It was suggested that the desorption activation energy was related with the thermal stability of surface groups on the activated carbons.The oxygen and sulfur-containing functional groups have greatly affect the adsorption stability of Cr（VI）via chemical adsorption.