Immobilization Of Soil Heavy Metals Using CaO, Fly ASH, S And Na₂S

Heavy metals(HMs) contaminated soils are serious threat to human health. Over the years, different soil amendments have been used to access the HMs immobilization/stabilization performance in polluted soils through short-term incubation methods. However, there is an increasing need to test long-term effects of soil amendments on HMs fixation through prolong incubation in view of the ecological engineering practice. Therefore, the aim of this study was to evaluate the effectiveness and aging factor of Ca O, fly ash, sulfur and Na2 S on the immobilization of Cd, Cu and Pb in three different contaminated soils under one-year incubation. The bioavailability of Cd, Cu, and Pb in treated soil samples were assessed by the Chinese cabbage through pot experiment. Hereafter, the three study sites(S) located in Fengxian(Shannxi province), Chenzhou(Hunan province) and Kunming(Yuannan province) are termed as S1, S2 and S3, respectively. The collected soil samples were mixed with amendments in four desired treatments(T1, T2, T3, and T4) and incubated for one year under room temperature. The TCLP-extractable HMs and soil p H and EC were analyzed in treated soils after 2, 6, and 12-month incubation in first experiment. After one-year incubation, the treated soils were used for the pot experiment(45 days). The DTPA-extractable HMs in soil, total HMs content in root and shoot, p H and EC, plant biomass and chlorophyll content were analyzed after pot experiment. The results of the study can be summarized as below:(1) The higher concentrations of Ca O and FA treatments(T3 and T4) have significantly immobilized Cd in used soils. In addition, sulfur treatments have moderately immobilized Cd with some increasing tends as compared to control in S1 and S3. Furthermore, Na2 S treatments have significantly immobilized Cd particularly in S3.(2) The mobility of Cu content has increased due to addition of CaO treatments in S2 and S3 as compared to control. In contrast, Ca O treatments have effectively immobilized Cu in S3 as compared to control. T1 and T3 of Ca O have significant effect in decreasing theCu content in S3. Moreover, FA treatments have also impressive impact in reducing Cu content in used soils throughout the incubation period especially in case of S3 followed by S1 and S2. The sulfur treatments have moderately immobilized Cu content except T3 and T4 in S2 where slight Cu increase was noted as compared to control. Furthermore, Na2 S treatments have also significant impact in reducing Cu content as compared to control in the entire 12 M incubation period. T1, T2, T3 and T4 of Na2 S have demonstrated similar significant effect of Cu immobilization in used soils.(3) The Ca O treatments have similar trend of significant Pb immobilization particularly by T3 and T4 in S2 and T1 and T2 in S3, respectively. In case of S1, Ca O treatments have showed steady effect on Pb immobilization irrespective of application rates during whole incubation. Likewise, FA treatments have significant effect on Pb immobilization until 2 M incubation but afterwards they gradually increased the Pb mobility in used soils. Furthermore, sulfur treatments have demonstrated effective impact in decreasing Pb content in S1 and S3 except S2 where they have increased Pb mobility(T3 and T4). Moreover, Na2 S have showed significant impact in decreasing Pb content in used soils as compared to control. Na2 S treatments(T1, T2, T3, and T4) have demonstrated stable effect on Pb immobilization in S1 and S3 whereas T3 and T4 have more significant effect then T1 and T2 in S3.(4) The results of pot experiment demonstrated that the concentration of DTPAextractable Cd was significantly decreased except in sulfur treated samples where a moderate increase of Cd content was observed in S1 as compared to control. In addition, Cu content has shown different behavior and moderate increase was noticed in S1 and S2. However, Cu content was significantly immobilized in S3. Likewise, Pb content was also efficiently immobilized by used amendments particularly in S3. In general, the solubility of HMs has increased at greater extent in soil system and shoots of Chinese cabbage have shown huge potential in uptake of Cd, Cu, and Pb. The treatments of Ca O and FA have demonstrated significant shoot uptake of Cd in S1 followed by S2 and S3 as compared to control. Meanwhile, Na2 S treated S3 has shown great capacity for Cd uptake in shoots as compared to control. The used soil amendments have increased Cu uptake in shoots in used soils as compared to control. However, a moderately increased Pb uptake was noted in shoots in used soils as compared to control. In contrast to HMs uptake in shoots, theroots of Chinese cabbage have also shown great capacity to uptake Cd, Cu, and Pb contents as compared to control. The roots have demonstrated significant Cd uptake in Ca O and FA treated samples of used soils as compared to control. A slight to moderate Cu uptake has been observed in roots as compared to control. However, the roots have shown great potential to uptake Pb content in used soil as compared to control. The sulfur treatments have put negative effect through decreasing p H of used soils except S1. Due to this reason, Chinese cabbage could not be grown in sulfur treated samples of S2 and S3. This study assumed that if sulfur amendment would be applied for immobilization of metals in alkaline soils rather than acidic soils then it might give better immobilization results. In order to achieve sustainable phytoextraction of Cd, Cu, and Pb using Ca O, FA, and Na2 S, the non-edible hyperaccumulators or normal plant species are suggested to be investigated in future studies. Using alkaline amendments, this ecologically risk may be controlled through testing energy crops for the purpose of phytoextraction instead of growing edible crops in the research studies of near future. In general, we assume that our study findings would be help soil scientists in decision making process to apply alkaline amendments for purpose of extracting Cd, Cu, and Pb using vegetable crops in contaminated soils across the world.