| Nanfeihe is the principal river flowing through Hefei City from west to east, then emerging into Chaohu Lake. Almost all industrial wastewater and domestic sewage along the river bank were discharged into Nanfeihe, leading to all kinds of pollution, especially heavy metals, that damaged water body and superficial sediment seriously. Based on the solidification/stabilization technology, several modified clay minerals were employed to carry on the harmless treatment on the contaminated sediment, so as to relieve the stress of water system and reduce heavy metals pollution. Solidification refers to techniques that encapsulate the waste, forming a solid material and does not involve chemical interaction between the contaminants and the solidifying additives. Stabilization is a remediation technology used to convert the toxic fractions into less soluble or more stable fractions by adding additives, so as to chemically reduce the bioavailability and mobility of contaminants. Bentonite, diatomite and sepiolite were employed as the stabilizers after being modified with organic, inorganic and hydrochloric acid, which were named as organic-modified bentonite, manganese-modified diatomite and acid-modified sepiolite separately. In addition to the increase of surface areas, the modifications brought out the enlargement of inter-layer spacing for bentonite, the enhancement of electronegativity and the formation of manganese hydroxide for diatomite, the elimination of impurity, the open of porous channel and the increase of Si-OH for sepiolite. In this way, the characteristics of clay minerals were improved in terms of adsorption performance.The sediments were sampled from the channel segment of Changjiang Road Bridge, and preliminarily treated for following two-stage experiments. Stage one referred to adding the stabilizer directly into naturally contaminated sediments at a mass ratio of 10:100. The mixtures were cured for two months and samples were taken at intervals. Toxicity leaching test and BCR sequential extraction experiment were implemented to study the impact of modified clay minerals on the release of heavy metals and fractions distribution. High-concentration heavy metals solution were synthesized first in the lab at stage two, which then was followed by the same procedure of stage one. Thus, it was investigated that the effect of stabilizers acted on the release of heavy metals and fraction distribution for heavily contaminated sediment. Contrast tests were implemented by using original clay minerals instead of modified clay minerals at each stage to check if the adsorption performance of clay minerals on heavy metals were improved after modification.Conclusions were obtained as follows:BCR sequential extraction method was utilized to test the fraction content of Cu, Pb, Cr in sediment samples. The total content of Cu, Pb, Cr was calculated as166.63 mg·kg-1, 36.84 mg·kg-1,249.15 mg·kg-1, all of which exceeded the figures of local soil background values and geochemical baseline values and morn than 50% were bio-available fractions with much biological risk.The naturally contaminated sediment showed low-concentration leaching liquor and the leaching concentration was below 1.2mg·kg-1 extracted by distilled water, while lower than 3.2mg·kg-1 by 1# reagent. MB, MD, MS indicated obvious inhibitory action on Pb and Cr at neutral condition, with reduction percentages of 68.67%, 26.67%, 42% and 16.67%, 20.83%,18.33% respectively. The three stabilizers showed different inhibition on the release of Cu, Pb, Cr as well, with inhibition ration of 31.79%, 39.749%, 19.87% and 10%, 30%, 10% respectively. The outcome of fractions distribution revealed that MB, MD and MS were feasible to be used for the remediation of sediments contaminated by Pb and Cr, especially with a considerable superiority on Pb contamination. Much of F1 and F2 was converted to F3 and F4, and the sum of the latter two fractions increased by over 75%. In a word, MB, MD and MS were good stabilizers to ameliorate lightly pollution sediment contaminated by Pb and Cr.The heavily contaminated sediments were extracted with extremely high leaching concentration of heavy metals. Cu got the highest inhibition ratio remediated by MB, with a data of 30.09% at neutral condition and 22.54% at acid condition. Cu, Pb and Cr extracted from the samples stabilized by MD were less than the blank samples, with significant inhibitory action on Pb and Cr, which were proved by inhibition ratios of 30.99% and 33.97% at neutral condition, and 60.71% and 51.05% at acid condition respectively. Only Cu was remediated in MS group, showing much lower inhibition ratio of 19.67% and 14.94 respectively. The data of fractions distribution revealed that F1 of Pb and Cr in MD group were decreased by 51.98% and 36.96%, and other groups showed weak change in fractions. In conclusion, MD had better amelioration performance on Pb and Cr for heavily contaminated sediment.The contrast tests indicated that the mobility of heavy metals for modified clay minerals was reduced. In another word, clay minerals after modification were improved in terms of adsorption performance on heavy metals. Because of excessively low leaching content of heavy metals at stage one, modified clay minerals cannot exert their advantages of adsorption completely. While MB and MS showed more advantage of inhibition on Cu and Pb at stage two, and MD manifested better result on restraining the release of Cu, Pb and Cr, in which Pb was significantly decreased. |
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