| As the world’s major leather processing country,China produces about 1 million tons of tannery sludge（dry weight）every year,with a chromium content of 1～4%.At present,landfill is still the main disposal way of tannery sludge,and the toxic and harmful substances such as chromium must be stabilized before landfill.In the field of research on the stabilization of heavy metal chromium,studies focusing on the synergistic stabilization step by step for heavy metal chromium are rarely reported,espeicialy with organic compounds and inorganic substances.Tannery sludge was considered as the sample,taking the advantage of high endogenous organic matter,together with adding inorganic stabilizer of non-toxic and harmless to study the mechanism on stabilizing chromium step by step.The main results were presented as follows:（1）Complexing effect of organic matter on chromium via forming Cr-O and Cr-OHCollagen protein waste（CPW）was used as exogenous organic matter to verifiy the complexation between organic matter and chromium.The single-factor experiments showed that the concentration of total chromium（Ccr）was determined to be 10 mg/L in the leach fluid.And Design of experiment（DOE）suggested the importances of fators was（A）time>（B）the particle size of tannery sludge>（C）the dosage of CPW.Infrared characterization（FT-IR）and X-ray electron spectral characterization（XPS）revealed organic matter and Cr was complexed via forming Cr-O and Cr-OH.DFT simulation was employed to verify the fact that Cr and oxygen was chemical bound and electron was transferred from oxygen to Cr,and the O 2p orbit and Cr 3 d orbit were overlapped within-2.5 e V～2.0 e V,-2.0 e V～2.0 e V and-1.0 e V～3.0 e V,respectively,for the three models.（2）MMT stabilizing exchangeable chromium via chemical adsorptionMontmorillonite（MMT）was added to reduce the proportion of exchangeable chromium（Ecr）in tannery sludge,which was changed from 33.32%to 0.30%（5.94 mg/L）.（DOE）suggested the importances of fators was（A）the dosage of MMT>（C）t water content>（B）the particle size of tannery sluge>（D）curing time.Cr-O,Cr-OH and Si-O-Cr peaks were observed using FT-IR.XPS showed that the peak area of free chromium was reduced from32.6%（raw tannery sludge）to 7.80%.DFT calculation verifieied the mechanism on cooperative stabilization:（1）interactions between MMT and organic matter:MMT-COC binding energy was-13.43052 e V.（2）Cooperative stabilization between organic matter and MMT:the binding energy of MMT-Cr-COOH ternary model（-77.36503 e V）was greater than the binding energy of MMT-Cr binary model（-61.29942 e V）（3）Cr and O atoms forming chemical bond（from oxygen-bearing functional groups and Si-O units）:the distance was close to the theoretical length of Cr-O after model optimization.（3）Phosphate stabilizing chromium of reducible and oxidizable state via forming Cr-O-PPhosphate stabilizer（sodium polyphosphate or sodium dihydrogen phosphate）was added to the sample,and the BCR sequential extraction experiment showed that the proportion of reducible and oxidable chromium（ROcr）was increased to 54.49%（Sodium polyphosphate）and65.89%（sodium dihydrogen phosphate）,respectively.Peaks of Cr-O-H,Si-O-Cr and Cr-O-P were observed in the result of FT-IR.And the results of XPS showed significantly reduced characteristic peak area of Ca-phosphate,suggesting that chromium ion exchanged with Ca and then it was fixed inside the phosphate lattice;Cr-O-P characteristic peak was also observed,indicating that Cr formed chemical bond with O atom in phosphate.DFT simulations verified that HPO42-and H2PO4-played a role in the stabilization process.On the one hand,it deionized-NH3 on organic matter,on the other hand Cr-O-P chemical bond was created between the O atoms and Cr.That was,the Cr 3 d orbital overlapped with the O atoms in P=O near 0 e V.（4）Exchaging with calcium and forming residual state chromiumBased on the above results,calcium-containing stabilizers were also introduced to the tanning sludge.The results of BCR showed that the residual chromium was increased from28.86%（original leather sludge）to 45.65%（sodium polyphosphate+calcium oxide）,55.76%（sodium dihydrogen phosphate+calcium oxide）,61.04%（calcium magnesium phosphate fertilizer）and 45.60%（calcium superphosphate）,respectively.FT-IR characterization results showed absorption peaks such as Si-O-Cr and Cr-O-H;XRD results speculated that Ca O-Al2O3-Si O2-H2O（C-A-S-H-H）and Ca O-Si O2-H2O（C-S-H）were produced;XPS results showed the peak areas of Ca-Phosphate and Ca-OH were increased.DFT simulation calculations verified that Cr and O were able to form chemical bonds and specified the positions of O atoms bonded to Cr.It was speculated that the proportion of effective oxygen in sodium dihydrogen phosphate-calcium oxide and calcium magnesium phosphate fertilizer was 66.7%,while the proportion of effective oxygen in calcium perphosphate was 100%.In conclusion,this study was conducted to evaluate the effects of endogenous organic matter,MMT,phosphate and calcium on improving the stability of chromium.The interactions among oxygen containing functional groups,Si-O,H2PO4-,Ca2+and Cr was analyzed.The relationships between organic matter and inorganic stabilizers were revealed,then the mechanisms on synergetic stabilization with organic and inorganic stabilizers were discussed.Based on the whole results,the formula which was MMT+calcium-magnesia phosphate fertilizer,was suggested to be more suitable for stabilizing chromium step-in-step,associating with endogenus organic matter.Therefore,it provides theoretical guidance for landfill pretreatment of tanning sludge,and protects agriculture soil from pollution via reducing the accumulation of chromium in soil. |
