جستجوی مقالات مرتبط با کلیدواژه « تثبیت فلزات سنگین » در نشریات گروه « آب و خاک »

Heavy metals such as lead, aluminum, mercury, copper, cadmium, nickel, and arsenic are now commonly found worldwide. Among these, cadmium and lead are the most hazardous, posing significant risks to both the environment and human health. Cleaning soils contaminated with organic and inorganic contaminants is one of the most significant and fundamental challenges facing society today. One effective method for soil purification is to extract or immobilize the contaminant within the soil.

It is unclear how water-soluble polymers contribute to the immobilization of heavy metals. The purpose of this study is to examine how various polymers affect the immobilization of lead, zinc, and cadmium in the soil near a lead and zinc mine in the province of Zanjan. A factorial experiment with three replications was conducted using a randomized complete block design. The experimental treatments included one type of soil and three different kinds of acrylic polymers (cationic, nonionic, and anionic) applied at four different levels (0, 0.05, 0.1, and 0.2). The absorbable amounts of lead, zinc, and cadmium were tested at various intervals after the polymers were applied to the soil samples. After that, SAS statistical software was used to examine the data. To do this, the Duncan multiple range test was used to compare the means. The necessary tables and graphs were then created using Excel.

The findings demonstrated that, at 1% probability level, the kind of polymer had a considerable impact on the amount of lead, zinc, and cadmium that may be absorbed in the soil. The average concentration of soil-absorbable lead for the different types of polymers employed was 239.8, 260.15, and 267.65 mg/kg; anionic polymer had the lowest concentration. Stated differently, anionic polymer decreases the capacity to absorb lead and stabilizes more lead in the soil than the other two forms of polymer. Anionic polymers most likely have a stronger impact on soil granulation. Additionally, at 1% probability level, the impact of acrylic polymer intake on the amount of lead, zinc, and cadmium absorbable in the soil was considerable. With an increase in the amount of polymer utilized in the soil, the greatest absorbable lead concentration (301.58 mg/kg) in the control treatment dropped to the lowest absorbable lead concentration (0.2). It was possible to determine the polymer percentage and the lead concentration, which came out to be 205.9 mg/kg of soil. Zinc concentration dropped as acrylic polymer consumption increased; in the control treatment, absorbable zinc concentrations ranged from 0.2 to 83.5 mg/kg of soil, with 0.2 being the highest concentration. At 1% probability level, the impact of the polymer's contact time with the soil on the amount of lead, zinc, and cadmium that the soil may absorb was significant. As a result, the tested soil had 414.52 mg of these elements at the initial stage of polymer treatment. The quantity of absorbable lead in the soil became 66% immobilized after a month, and after 720 hours, the amount of absorbable lead dropped to 141.83 mg/kg. As the polymer's contact time with the soil increased, so did the concentration of absorbable zinc in the soil. At 1% probability level, there was a strong correlation between the kind and amount of acrylic polymers and the amount of lead, zinc, and cadmium that may be absorbed in the soil. The ingestion of 0.2% anionic polymer resulted in the largest amount of lead immobilization, lowering the soil's absorbable lead concentration from 300 to 192 mg/kg of soil. A higher amount of anionic polymer immobilized the lead, and both cationic and non-ionic polymers were positioned after it. Additionally, anionic polymer was more prevalent than cationic polymer. It caused the non-ionic polymer's absorbable zinc to become immobile. Following 720 hours of polymer treatment, the soil's absorbable zinc element was immobilized to a greater extent by the anionic polymer (20%) than by the cationic and non-ionic polymers (26%), respectively. In comparison to the original concentration, the largest amount of immobilization by anionic polymer after one month was 78%, and the lowest amount of immobilization by nonionic polymer was 61%. Anionic polymer was 27% more effective than non-ionic polymer, 18% more effective than cationic polymer, and stabilized more cadmium.

 The results of this study showed that with increasing the duration of contact of polymers used with the soil, the amount of mobility of heavy metals in the soil decreased and also with increasing the amount of polymer consumption, the rate of metal stabilization in the soil increased. Anionic polymers immobilize more lead, zinc and cadmium in soil. To reduce the mobility of lead, zinc and cadmium and improve the stability and increase aggregation in soil, the use of acrylic polymer in contaminated soil is recommended.

The use of biochar as an amendment in soils contaminated with heavy metals is an economical and environmentally-friendly method. The biochar's effectiveness in heavy metals stabilization is related to the biochar and soil properties. Success of biochar application is related to determining the most important characteristics of biochar that increase effectiveness of biochar. It is not easy to use a wide range of characteristics in a study, therefore the most appropriate results are obtained by analysis the results from different studies. Meta-analysis is the application of statistical techniques to combine and summarize the findings of multiple studies. By combining data from many studies, meta-analyses can provide more precise information about biochar. Therefore, in the present study, the effectiveness of biochar for remediation of heavy metal contaminated soils in Iran has been studied by meta-analysis and the most important factors were determined.

The articles were selected from various databases based on keywords such as bioavailability, various heavy metals forms, biochar, remediation of contaminated soils, heavy metals stabilization, in the of period 2016-2020. The data were extracted from results of 30 articles. Finally, the required information was collected from 11 articles and 123 independent observations. In these studies, the evaluation of heavy metals bioavailability was carried out with different methods, such as mobility, leaching, adsorption kinetics, plants bioavailability and heavy metals bioavailability in soil.  In order to determine the effectiveness of biochar, the size effect variable (R) was estimated for the bioavailability differences between the control and treatment groups. Due to, there were different methods for assessing the biochar effectiveness, in this study, the biochar effectiveness was calculated by obtained information from bioavailability studies. Also, Spearman correlation test was used to examine the relationships between variables. Statistical analysis was performed using SPSS and Excel software. Also, Web Plot Digitizer software was used to extract data from the presented graphs in the articles.

Based on the results from most studies, agricultural wastes were used as the raw materials for biochar production. In these studies, the treatments such as temperature, pyrolysis time, weight percent of biochar in soil, and incubation time were used. The weight percent of biochar in soil was more considered than other treatments by researchers. In the case of soil, variables such as pH, EC, CEC, CCE, soil texture, organic matter were more considered than other physical and chemical properties. There was a statistically significant difference between the studied heavy metals in terms of R variable (p <0.05). According to the results from studies, biochar was more effective in decreasing the bioavailability of soil lead. Also biochar was most effective in reducing the bioavailability of zinc and cadmium. Effectiveness of biochar had significant positive correlation with biochar pH, application rate in soil, incubation time, pyrolysis temperature and carbon content of biochar, the amount of silt and clay in soil, soil EC, soil CEC and soil organic matter but significant negative correlation with biochar EC, amount of sand and soil pH. The highest correlation coefficient (0.515) was found for soil CEC. Therefore, this factor was the most important factors influencing the effectiveness of biochar in stabilization of heavy metals.

In Iran, agricultural wastes have higher priority for biochar production, because the high rate of waste prodution in agriculture. On the other hand, there are processing problems for other raw materials, including the high concentration of heavy metals in wastewater, high cost and need for advanced equipment to obtaion a secure biochar for application in the soil. Based on the obtained results, biochar is effective in stabilizing heavy metals in contaminated soils and in this regard, there is a positive or negative relationship between soil properties and biochar. Therefore, it seems that by improving some factors that have a positive correlation, the effectiveness of the biochar can be increased, and for some characteristics that have a negative correlation, the effectiveness of the biochar can be improved by modifying them. In these studies, the long-term effects and security of biochar applications for soil remediation, soil organisms, and plant growth need to be considered.