Comprehensive Utilization Of Alkaline Residue And FGD Gypsum For Immobilization Of The Heavy Metals

With the rapid development of China's national economy,the increasing level of solid waste are generated by industrial activities,such as soda ash production.coal power generation and so on.Due to their huge quantities and low utilization efficiency,a considerable portion of them is inappropriately disposed in open landfills,which has seriously restricted the sustainable development of related industries.In order to find more potential markets to improve the utilization efficiency of alkaline residue(AR)and FGD gypsum,the present study suggests an innovative utilization approach for AR or FGD gypsum which is the development of phosphorus containing materials from them for the remediation of heavy metals(eg.Pb,Cd)pollution.At first.AR was respectively modified by NaOH(AR-NaOH)and HCl(AR-HCl)to prepare the adsorbents,then the batch experiments were conducted to evaluate the sorption performance and sorption mechanism of obtained adsorbents for phosphate in aqueous solution.The results showed that the maximum adsorption capacities of adsorbents were in following manner:AR-NaOH(211.9 mg/g)>AR(134.8 mg/g)>AR-HCl(2.2 mg/g),which was same with the order of CaO content in each adsorbent.Moreover,the sorption mechanism of phosphate on AR-HCl was ligand exchange or hydrogen-bond interaction,while the formation of CaHPO4·2H2O was the main mechanism for AR and AR-NaOH.The results indicated that the high CaO content is in favor for phosphate adsorption.As we know,the CaO content in calcium carbonate rich materials can be enhanced by means of thermal treatments.In this case,AR was calcined at various temperatures between 400 and 1000 ?,and the highest phosphate sorption capacity was achieved at 800 ?(AR800).The results showed that the formation of Ca10(PO4)6(OH)2 was found after adsorption of phosphate on AR800.The feasibility of reusing the phosphorus loaded alkaline residue(after AR and AR800 sorption)as an amendment for immobilization of Pb in a shooting range soil was explored.The toxicity characteristic leaching procedure(TCLP).Tessier sequential extraction.and physiologically based extraction test(PBET)were employed to evaluate the effectiveness of Pb immobilization in soil.Treatment with phosphorus loaded alkaline residue was significantly effective in reducing the TCLP and PBET extractable Pb concentrations and phytoavailable Pb in contrast to the AR and AR800 treated soil.After treatment with phosphorus loaded alkaline residue,more than 60%of soil-Pb was transformed to the residual fraction compared to the original soil.Moreover,phosphorus loaded alkaline residue also resulted in a drastic reduction in phytoavailable Pb to the winter wheat and a mild release of P as a nutrient in treated soil,which also confirmed the improvement of soil quality.Interestingly,the amendment consisting of Ca10(PO4)6(OH)2 is more effective than the amendment with CaHPO4·2H2O in all cases.Based on the above results,the feasibility of using waste FGD gypsum as Ca source to prepare hydroxyapatite(FGD-HAP)was assessed.The obtained FGD-HAP was characterized by XRD,FTIR,TEM and BET methods and investigated as adsorbent for removal of Pb(?)and Cd(?)from aqueous solution.Batch experiments were performed by varying the soltion pH values,contact time,initial metal concentration and temperatures.The achieved experimental data were analyzed with various kinetic and isotherm models.The competitive adsorption study of Pb(?)and Cd(?)in binary system was carried out to know the preferential sorption performance of FGD-HAP.The mechanism involoved in sorption process was investigated via the combination of modeling,XRD,and TEM-EDS methods.The results showed that the surface of FGD-HAP was homogenous,the adsorption of Pb(?)and Cd(?)was pH dependent process,the optimum pH range was 5.0-6.0,the adsorption was endothermic and spontaneous in nature.The dissolution of FGD-HAP followed by re-precipitation of more stable Pb5(PO4)3OH is the dominated mechanism in Pb(?)adsorption process,while the exchange of Ca(?)with Cd(?)in lattice of FGD-HAP mainly contributed to Cd(?)adsorption.The FGD-HAP had the stronger binding affinity for Pb(?)than Cd(?).Moreover,the maximum adsorption capacities calculated from Langmuir equation were 277.8 and 43.10 mg/g for Pb(?)and Cd(?),respectively,which were even higher than other special prepared adsorbents,implying that FGD-HAP had a significant potential for removal of Pb(?)or Cd(?)in real wastewater.The feasibility of using FGD-HAP and FGD gypsum as amendments for immobilization of Pb and Cd in spiked soil was explored.The simulate acid rain extraction(SAR),CaCl2 extraction,BCR sequential extraction,and pot experiments were conducted to evaluate the effectiveness of Pb and Cd immobilization in spiked soil.The results showed that application of FGD-HAP and FGD gypsum induced significant reduction in SAR and CaCl extractable Pb concentrations and exchangeable and phytoavailable Pb in contrast to unamended soil.The decreased extractable metal content of soil was also accompanied by improved soil enzyme activity which significantly increased(P<0.05)in 5.0%FGD-HAP and FGD gypsum amended soils.Interestingly,the FGD-HAP is more effective than FGD gypsum in all cases,indicating that FGD-HAP has a great application prospect for in situ remediation of the soil contamined with Pb and Cd.This study not only provides a new bulk utilization way for AR and FGD gypsum,but also lays a theoretical basis for the environmental remediation applicaiton of other Ca-containing industrial by-products.