Studies On The Adsorption And Stabilization Characteristics Of Heavy Metals Onto The Sediments

Heavy metals released into the aquatic environment would translate from the liquid phase to the solid phase through physical, chemical and biologic process. Meanwhile, the heavy metals in the sediments would also release into the water via physical, chemical and biologic process when the conditions were changed, and the water would be polluted by the heavy metals again. So the sediments act as both sources and sinks for potentially harmful substances. The surface coatings widely forming on surfaces of rocks and sediments in the river, lake and wetland, are mainly composed of metal oxides (such as Fe, Mn and Al oxides), organic materials and some minerals, and could be classified as special sediments. Sediments (surface coatings) are believed to play significant roles in the cycling and bioavailability of trace metals. Within the sediments (surface coatings), the three most important geochemical components were identified as Fe, Mn oxides and organic material. To determine the speciation and sorption mechanisms of heavy metals in sediments (surface coatings), the correlation analyses, sequential extraction procedures and a combination of chemical extraction and adsorption techniques were used and the results of these studies were all qualitative. The selective extraction– adsorption– statistical analysis method has been proved to be an effective approach to quantitatively evaluate the contributions of Fe/Mn oxides and organic materials in the natural surface coatings developed on the on artificial substances, such as glass slides fixed on polypropylene racks. But the sediments and surface coatings developed on the natural substances, such as the sediments, were different from the natural surface coatings developed on the artificial substances in growth environment, composition or adsorption capacities. So the selective extraction applied to the natural surface coatings developed on the artificial substances would not be fitted to sediments (surface coatings). The one of the main objects of this study was to estabilish and perfect a selective extractive extraction technique applied to sediments (surface coatings), and then quantitatively evaluate the contributions of Fe/Mn oxides and organic materials in the sediments (surface coatings) via selective extraction– adsorption– statistical analysis method in order to further understand the adsorption mechanism of heavy metals onto the sediments (surface coatings). The heavy metals released into the water could be stabilized in the sediments through adsorption and precipitation, and then the threat of heavy metals in the sediments would be lowered. The transformation of heavy metals from the less stable phases of sediments to more stable phases of sediments via physical, chemical and biological process and consequential decreasing of cyclying of heavy metals in the sediments would be a potientally effective method to stabilize and immobilize heavy metals in the sediments. So, 11 kinds of organic materials, Fe oxides, clay minerals and phosphate minerals were chose as the poiental materials for stabilitating and immobilizing heavy metals in the sediments by a combination of the results of the above studies and the results reported in the former literatures. Study on the stabilization characteristics of heavy metals onto the sediments through the selected materials was performed. This will help further understand the stabilitation and immobilization process of heavy metals, and present a technical possibility for the disposal and cycle-use of heavy metals widely existed in the sediments.The sequential extraction results of heavy metals in the sediments (surface coatings) implied that the Fe and Mn in the sediments (surface coatings) were mostly exsited as residual phase and thus the determination of the total extractable amounts of Fe and Mn by using HNO3 of 10% or 15% was not fittable for Fe and Mn in the sediments (surface coatings). Meanwhile, the former studies indicated that the extraction reagents and conditions used in the selective extraction technique applied to the surface coatings developed on the artificial substances were not completely fittable for sediments (surface coatings). So, the selective extraction technique was established and perfected based on the former studies in order to selectively remove and separate Fe, Mn oxides and organic materials in the sediments (surface coatings). The details of selective extraction technique were as follows: The total extractable amounts of Fe and Mn were determined by a modified sequential extraction procedure; 0.1 mol/L NH2OH·HCl + 0.1 mol/L HNO3 was employed to remove Mn oxides, (NH4)2C2O2 (0.2 mol/L) + oxalic acid buffer solution (pH 3.0) was used to digest both Fe and Mn hydrous oxides, and 30% of H2O2 heated in the water bath of 40℃was applied to selectively extract organic materials; The separation of clay minerals and silicate minerials were compeled by using suspending and sedimentation. After the extraction treatments of the sediments (surface coatings) pretreated with different methods, the sediments collected at the same sampling time but in the different sampling locations and the sediments (surface coatings) collected in the same sampling location but at the different sampling time, the target components were extracted with efficiencies larger than 68%, and the non-target components of lower than 34% were removed, implying that this selective extraction technique was effective for the removal and separation of Fe, Mn oxides and OMs in the sediments (surface coatings), and important for further mechanism study of trace metals adsorption onto sediments (surface coatings).The selective extraction techniques and statistical analysis were used to investigate the roles of non-residual and residual components of sediments (surface coatings) in controlling Cu and Zn adsorption. The results indicated the metal adsorption abilities of surface coatings were much stronger than those of sediments, and the sdsorption capacity of Cu was much larger than that of Zn. The greatest contribution to metals adsorption on a molar basis was from Mn oxides in the non-residual fraction. Metals adsorption capacities of Mn oxides exceeded those of Fe oxides by one order of magnitude, few roles were found attributing to adsorption by organic materials and the estimated contribution of the residual fraction to metals adsorption was insignificant. These results implied that metals (Fe and Mn) oxides were the most important components in controlling heavy metals in aquatic environments. Cu and Zn adsorption onto the clay minerals separated from surficial sediment were also carried out. The results suggested that the retention of heavy metals by clay minerals was significantly greater than that of the surficial sediment. But compared to the other components in the surficial sediments (Mn oxides, Fe oxides and organic materials), the adsorption capacity of clay minerals was relatively lower on a unit mass basis, implying that the roles of clay minerals in the surficial sediments contributed to the accumulation of heavy metals were less than Fe/Mn oxides and organic materials. The surface area, infrared spectra, scanning electron micrographs and X-ray powder diffraction patterns of the sediments before and after extraction treatments were also investigated. The results indicated that the surface and particulate characteristics of the sediments after each extraction significantly changed, and the BET surface area increased, dispersing degree of the sediment particles enhanced, as well as more distinct of the crystal structure. But XRD and FTIR spectra changed slightly after each extraction due to the identified peaks were mainly contributed by minerals in the sediments. The adsorption capacities of the sediments after extractions decreased along with the dispersing degree increasing of the sediment particles, the coarse weakening of sediment surfaces, implying that the surface characteristics of the sediments also contributed to the adsorption of heavy metals to the sediments. And the negative correlation between the BET surface areas and the adsorption capacities of the sediments suggested that the adsorption contributions from the sediment components were significantly greater than those from surface and pore structure of the sediments.11 kinds of materials including organic materials such as tannin acid, chitosan and humic acid, iron oxides such as goethite, hematite, ferrihydrite and lean hematite, clay minerals such as illite and montmorillomite, phosphate minerals such as hydroxylapatite and diammonium hydrogenorthophosphate were chosen as the potential materials for stabilitating and immobilizing heavy metals in the sediments based on the above studies which indicated that the sorptive sites in the sediments were Fe and Mn oxides, organic materials, clay minerals and silicate residues, as well as the the results reported in the former literature. Lean hematite, tannin acid, montmorillomite and hydroxylapatite were initially selected as potential materials for the stabilitation and immobilization of heavy metals in the sediments by investigating the adsorption capacities of solid materials, the changes of the physical and chemical and adsorption characteristics of the sediments after inserting of the materials into the sediments. The fractions of heavy metals in the sediments indicated that the effects of lean hematite and montmorillomite inserted in the sediments on the distribution patterns of heavy metals in the sediments were lower, and the most transformation degree was less than 13%. Tannin acid inserted into the sediments lead to the transformation of Cu and Zn from carbonatic phase to exchangezble phase (>18%). Heavy metals bound to Fe/Mn oxides, organic materials of sediments were increasing after the sediments inserting with hydroxylapatite, and about 50% of Zn, Pb, Cd and 10% of Cu existed as exchangeable phase were translocated to other more stable phases. The stability of heavy metals in the sediments was also increased after the the sediments inserting with lean hematite, tannin acid, montmorillomite and hydroxylapatite as proved by the results of TCLP extraction. The effect of hydroxylapatite was the most obvious and 50% of Cu, Zn, Cd and more than 82% of Pb were immobilized in the sediments as compared with the sediments without inserting hydroxylapatite. So, hydroxylapatite was recognized as the most fitable materials for stabilizing and immobilizing heavy metals in the polluted sediments.The results iof heavy metals and P release experiment suggested that the less low of pH, the more release of heavy metals. The release amounts of heavy metals were obviously decreased as the inserting of hydroxylapatite, and this phenomena was most obvious for Cu and Pb when pH was at 3 or 4 and at 5 or 7 for Cd. The released amounts of P reached peak when the pH value of suspended solution was 3. The fractions of heavy metals in the sediments after the inserting of hydroxylapatite at different pH values implied that the heavy metals present in exchangeable phase and carbonatic phase of sediments were translocated to more stable phases, such as Fe/Mn oxides phase, organic phase and residual phase. The transformation of heavy metals in the sediments after the inserting of hydroxylapatite at pH 4, and 15.9% of Cu, 30.8% of Pb, 12.2% of Zn and 37.2% of Cd have been translocated from exchangeable phase and carbonatic phase to the other more stable phases. Cu was mostly transformed to organic phase, Pb was to organic and Fe/Mn oxides phases, Zn was to organic and residual phases, and Cd to Fe/Mn oxides phase. The results of TCLP extraction and bioavailability extraction suggested that the stabilization effect of hydroxylapatite for heavy metals in the sediments was the most valid at pH levels of 4-5, and not obvious at pH 7. When pH was 3 or 9, the amounts of heavy metals extracted from the sediments by TCLP, CaCl2 or MMS extraction solutions increased after the inserting of hydroxylapatite, implying that the toxicity and bioavailability of heavy metals in the sediments after the inserting of hydroxylapatite enhanced. Heavy metals were stabilized and immobilized via dissolution and precipitation as investigated by XRD, FTIR, SEM, energy spectrum, and fraction analysis. The effect of pH on the stabilization of hydroxylapatite was more important, and the most stabilization effect was reached at pH levels of 4-5.