Stabilization Treatment Of Heavy Metal Contaminated Solid Wastes/Soils By Ferrous Salt

Ferrous salts have been widely applied for the stabilization of heavy metals in solid wastes,soils and water,since they are economical,environment-friendly and broadly available.Generally,the stabilization of heavy metals in solid wastes and soils are more complicated than that in water,due to the complexityof solid phases.In this dissertation,we applied ferrous salts to the stabilization treatment of various solid wastes/soils contaminated by heavy metals,so as to investigate the effect of those objects'intrinsic properties on the stabilization of heavy metals.The main contents of the dissertation were summarized as follows:?1?Ferrous salts were used for the stabilization treatment of chromite ore processing residue?COPR?,illuminating the effect of COPR's intrinsic properties on Cr?VI?stabilization.Based on the Cr?VI?leaching concentrations of treated COPR samples curing for 7 and 400 days,FeSO₄/H₂SO₄ treatment was more efficient than FeSO₄treatment.More importantly,the FeSO₄/H₂SO₄ treated COPR showed better long-term stability.The difference was caused by Cr?VI?in Cr?VI?-bearing minerals,which was not available for Fe?II?due to mass transfer limitations.For FeSO₄/H₂SO₄ treatment,the release of Cr?VI?from Cr?VI?-bearing minerals was enhanced making Cr?VI?easier accessed by Fe?II?.Based on the results of independent leaching tests,both acid dissolution of Cr?VI?-bearing minerals and sulfate anion exchange with chromate were responsible for the enhanced release of Cr?VI?.XRD,XPS and alkaline digestion analyses were also employed to interpret the possible stabilization mechanism,showing that Cr?VI?released from COPR solid was reduced to Cr?III?by Fe?II?and then formed stable Fe_xCr?1-x??OH?₃ precipitate.?2?Ferrous salts were applied for the stabilization treatment of As-bearing solid wastes,including arsenic-alkali residue?AAR?from antimony smelting and arsenic-bearing sludge?ABS?from sulphuric acid plant.The different effects of the two objects on As stabilization by ferrous salts were focused.For AAR stabilization treatment,we found that ferrous salt treatment was more efficient than lime treatment,because only one-tenth of dosage of lime was required for ferrous salt to reduce As leaching concentration of AAR to lower than 2.5 mg/L.According to the results of XRD,XPS,As species analysis and chemical equilibrium modeling,it was found that the obvious difference was mainly depended on two properties of AAR,i.e.As species and coexisting soluble carbonate.The main As species in AAR was soluble arsenate,which made the formation of insoluble arsenate minerals being the dominated mechanism of As stabilization.On the other hand,the coexisting soluble carbonate had little effect on Fe?II?-As?V?mineral,but it could obstruct the formation of Ca-As?V?mineral.For AAR stabilization treatment,the stabilization mechanism of As was much different from that in AAR,because the species and valence state of As in the two As-bearing solid wastes were entirely unlike.As?III?bound to calcium was the major component in ABS.Based on the results of XRD,XPS and As species analysis,the main stabilization mechanism of As in ABS included:?1?the transformation of soluble As and As bound to calcium to As bound to iron oxides;?2?the oxidation of As?III?to As?V?due to the molecular oxygen activation by Fe?II?.As a conclusion,the intrinsic properties of As-bearing solid wastes had a marked impact on the As stabilization effects and mechanisms.?3?Ferrous salts were applied for the stabilization of As in As-bearing kaoline to investigate the role of kaoline.By comparison,either kaoline?K?system or Fe?II?system were unable to reduce the As concentration of filtrate passed though 0.45?m membrane,whilst the As concentration of filtrate reduced with the oxidation of Fe?II?in K/Fe?II?system.This phenomenon implied that kaoline participated in the reaction between Fe?II?and As?V?under aerobic conditions.According to the results of zeta potential,Tyndall effect,TEM and DLS analysis,we found that?a?kaoline did not adsorb As due to its surface with negative charges;?b?in Fe?II?system without kaoline,the reaction products were nanocolloidal Fe?III?-As?V?particles with 30-300 nm diameter,which had high mobility;?c?in K/Fe?II?system,the products were micron-sized K-Fe?III?-As?V?particles with 3-7?m diameter,which were easy to settle and filter.Because of the participation of kaoline,the homogeneous reaction between Fe?II?and As?V?transferred to be heterogeneous reaction,resulting in the distinction of products.Moreover,aqueous As,nanocolloidal Fe?III?-As?V?and micron-sized K-Fe?III?-As?V?were separated to reveal the effect of kaoline content and pH.It shown that both kaoline content and pH affect the proportion of homogeneous and heterogeneous reactions,whilst the As/Fe mole ratio in solid products was only controlled by pH.