Study On Interference Of Ferrous Ions In Leaching Toxicity Analysis By Pre Aeration Method

Chromium is one of the most common heavy metal contaminants in soils,which mainly results from chromium-containing wastewater/sludge and chromite ore processing residues(COPR).Researches on treatment of chromium-containing wastes and remediation of chromium-contaminated soils are prevalent now.The most common technique is to reduce hexavalent chromium into low toxicity trivalent chromium,and the most preferred reductant is ferrous.However,due to many factors,hexavalent chromium in solid matrices usually cannot be reduced completely,resulting in a large quantity of unreacted reductants left in solid matrices.Even after a long period of curing,the residual reductant would remain to a certain degree.This residual reductant might reduce hexavalent chromium when analyzing leaching toxicity,resulting in significant bias.Some studies have reported that the leaching concentrations of Cr(?)always increased with curing time,and put forward long-term stability issue in the remediation of chromium-contaminated soil.With the three most common toxicity characteristic leaching procedures(TCLP),this thesis explored the influence of residual ferrous on Cr(?)leaching concentrations on the ferrous-treated COPR and chromium-contaminated soils,and further proposed alkaline-aeration pretreatment to mitigate or eliminate the effect of residual ferrous.With improved leaching procedures,the mechanisms of long-term stability in the remediation of chromium-contaminated soils were explored.Based on the three-layer structure for reduced chromium-contained solid matrices,an alkaline-aeration pretreatment procedure is proposed.Experimental results show that the residual ferrous in reduced COPR and chromium-contaminated soils can reduce the Cr(?)in the leaching solution,resulting in the negative bias.The magnitudes of the negative bias can be monitored by before-TCLP spiking.In the initial stage of leaching operation,alkaline aeration can effectively remove the residual ferrous,resulting in the improvement of before-TCLP spiking recoveries and meeting the quality control requirements for analysis data.For the long-term stability issue,the comparisons of Cr(?)leaching concentrations between TCLPs and alkaline-aeration TCLPs prove that the increase of Cr(?)leaching concentrations with curing time is caused by analyze bias.The actual Cr(?)leaching concentrations were far higher than the analysis results with serious bias,and decreased with curing time.