Systematic review and meta-analysis on the effectiveness of biochar for remediation of contaminated soils by heavy metals in Iran and determination of effective factors

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.