Studies On Immobilization Of Heavy Metals By Hydroxyapatite In Contaminated Sediments

People in the world focus attention upon the problem of heavy metal pollution in the environment, it urgently needs to seek a simple, economical effective, in situ and relatively mature technique in chemical remediation, phosphorous containing material remediation has merits including in situ, economical, effective et al. Therefore, much more attention should be paid to this remediation techniqueHydroxyapatite (HA) was used to immobilize heavy metal pollution in sediments obtained from Huludao. Single extraction procedure and Tessier's sequential extraction procedure were used to assess the bioavailability of heavy metals in sediment with HA. 0.01mol/L CaCl2, MMS and other two extractants referred in US EPA toxicity characteristic leaching program TCLP1#, 2# were chose as extractants of the single extraction procedure. Furthermore, to study the immobilization mechanisms of heavy metal and the release of heavy metals and phosphor, we investigate the release of heavy metals and phosphor during remediation process at different pH value (3, 4, 5, 7 and 9). Eventually, we try to analyze the immobilization mechanism utilizing the XRD, SEM, and FTIR combine with the partitioning patterns of Cu, Zn, Pb and Cd. The results indicated that:After investigation of the bioavailability of heavy metals in sediment using 0.01mol/LCaCl2, MMS and TCLP two extractants 1#, 2#, we find the following results: 0.01mol/LCaCl2, MMS and TCLP two extractants 1#, 2# were feasible as extractants to study the bioavailability of heavy metal in sediment, it well assess the mobility of heavy metal in sediment at same pH. Certainly, there were some deficiencies: the conclusions of four extractants were variant at high pH, especially 0.01mol/LCaCl2 and MMS. They were incapable of being applied to assess the bioavailability of heavy metal in neutral and alkalescent sediment.The immobilization effect of Cu, Zn and Cd were visible when pH was 4 and 5, the average reduction of the bioavailability of them reached Cu26% and 31%, Zn13% and 20%,Cd26% and 31%, respectively compared with control,after 90d remediation by HA, the effect of Pb was the best at pH 4 with the bioavailability of Pb reduced about 80%.Sediment was remedied at four rates of HA addition and the results of using TCLP 1# and 2# show that bioavailability all depressed and the result using TCLP 2# revealed that there was positive correlation between reduction of four heavy metal's bioavailability and the quantity of HA addition, the relativity between the reduction of Zn and Cd's bioavailability the quantity of addition HA was significant.The extraction result using 0.01mol/L CaCl2 and MMS showed that there was negative correlation between the reduction of extractable Cu, Zn and Pb and the quantity of HA addition. Maybe it is because CaCl2 and MMS can enhance desorption of heavy metal ion, it illustrated that 0.01mol/LCaCl2 and MMS were incapable of being applied to assess the bioavailability of heavy metal in sediment remedied by neutral and alkalescent HA.The data of Tessier's sequential extraction procedure concluded that: at different pH value, Cu, Pb, Zn and Cd were mainly transform from exchangeable and carbonate-associated matters to manganese & iron oxides, organic materials and residual matters, after 30d remediation by HA. The transform were visible when pH was 4, there were 16%Cu, 31%Pb, 12%Zn and 37% Cd transformed from exchangeable and carbonate-associated matters to other stabile matters, Cu mainly transformed to organic materials matters, Pb to organic materials and manganese & iron oxides matters, Zn to organic materials and residual matters, Cd to manganese & iron oxides matters.The proportion of HA addition had small effect on the transform of Cu, but there was positive correlation between transform of Zn, Pb and Cd and the quantity of HA addition.To study immobilization mechanism and the release of heavy metal and phosphorus, we also investigated the release of heavy metals and phosphorus during remediation process at different pH value, we obtain the following results: In the time range of the experiment, We believe that release of Cu, Zn, Pb and Cd in sediment reach balance after 12h, regarding the average of the concentrations after 12h as release quantity of heavy metal. Release of heavy metal was changing greater under very acidic condition; release of heavy metal is lower under condition of neutral and alkaline condition. The addition of HA can reduced the release of heavy metal in sediment. And along with the remediation,the release of phosphorus can fall to low level.The mechanisms of heavy metal remediation by HA were studied by XRD, SEM, FTIR combine with change of partitioning patterns of Cu, Zn, Pb and Cd, we thought that in short-term<90d, the main mechanism of immobilization was precipitate, form low solubility mineral M10(PO4)6(X)2.