In situ immobilization of Cr and its availability to maize plants in tannery waste-contaminated soil: effects of biochar feedstock and pyrolysis temperature

Purpose Tannery waste-contaminated soil has a high amount of several toxic chemicals and heavy metals including chromium (Cr), which makes it unsuitable for agriculture practices. Majority of studies have reported the use of biochar (BC) as an amendment to restore contaminated soil. The efficiency of BC to immobilize Cr depends on its pretreatment and feedstock. This study aimed to investigate the potential of using BC for Cr immobilization and reducing its availability to maize plants in tannery waste-contaminated soil. The effect of BC on plant growth and heavy metal (Cr, Cu, Zn, Pb, Fe, and Mn) contents of maize shoots was also investigated. Materials and methods The soil was collected from landfill areas of Riyadh city (N 24 degrees 25', E 46 degrees 34'). BCs were produced by pyrolyzing jujube (Ziziphus spina-christi) leaves (JL) and manure (M) waste at 300 and 700 degrees C. Collected soil and produced materials were characterized physically and chemically by following standard procedure. A greenhouse pot experiment was conducted with unamended tannery waste-contaminated soil and soil amended with the obtained BCs (JLBC-300, JLBC-700, MBC-300, and MBC-700) at an application rate of 50 g kg(-1) and cultivated with maize (Zea mays L.). Before cultivation, soil-soluble Cr was measured in five leachate cycles. Dry matter and heavy metal (Cr, Cu, Zn, Pb, Fe, and Mn) contents of maize shoots were measured after harvesting. Results and discussion The results show that BCs produced at 700 degrees C showed the highest decrease in the concentrations of soil-soluble Cr and reduced its cumulative concentrations in soil leachates by about 93% and 59.0% for MBC-700 and JLBC-700 compared with the control soil, respectively. Overall, the highest decrease in cumulative soluble Cr was pronounced for MBC-700 followed by JLBC-700. For heavy metal contents in shoots, Cr concentrations were under detection limit in all treatments. However, BC amendments showed significant differences from the control for Cu, Mn, and Fe. Conclusions From findings, it could be concluded that application of BCs pyrolyzed at 700 degrees C (especially for MBC-700) could be used as an amendment for reducing Cr mobility in tannery waste-contaminated soil and may create favorable conditions for crop production.