Date palm-magnetized biochar for in-situ stabilization of toxic metals in mining-polluted soil: evaluation using single-step extraction methods and phytoavailability.

Mining activities provide a pathway for the entry and accumulation of various heavy metals in soil, which ultimately leads to severe environmental pollution. Utilization of various immobilizing agents could restore such contaminated soils. Therefore, in this study, date palm-derived biochars (BCs: produced at 300 °C, 500 °C and 700 °C) and magnetized biochars (MBCs) were employed to stabilize heavy metals (Cd, Pb, Cu and Zn) in mining polluted soil. Metal polluted soil was amended with BCs and MBCs at w/w ratio of 2% and cultivated with wheat (Triticum aestivum L.) in a greenhouse. After harvesting, dry and fresh biomass of plants were recorded. The soil and plant samples were collected, and the concentrations of heavy metals were measured after extracting with water, DTPA (diethylenetriaminepentaacetic acid), EDTA (ethylenediaminetetraacetic acid), and acetic acid. BCs and MBCs resulted in reduced metal availability and uptake, with higher fresh and dry biomass (>36%). MBCs showed maximum decrease (>70%) in uptake and shoot concentration of metals, as these reductions for Cd and Pb reached below the detection limits. Among all single-step extractions, the DTPA-extractable metals showed a significant positive correlation with shoot concentrations of tested metals. Thus, the synthesized BCs and MBCs could effectively be used for stabilizing heavy metals and improve plant productivity in multi-contaminated soils. However, future studies should focus on long term field trials to restore contaminated mining soils using modified biochars.

This study has demonstrated the performance of magnetized biochars for in-situ stabilization of toxic metals (Cd, Pb, Cu and Zn) in mining polluted soil by single extraction method. All the produced BCs and magnetized BCs showed great potential in immobilizing the metals and reducing their availability in soil, consequently decreasing their shoot concentration and plant uptake. Significant negative correlations were observed between soil pH and metal extraction from applied extraction methods such as water soluble, DTPA, and EDTA extractions. We found DTPA as a suitable extractant for investigating metal uptake in plant in multi-contaminated soils. Treatments with MBCs showed maximum decrease in plant uptake and concentration of studied metals. Thus, application of MBCs could efficiently immobilize soil heavy metals.

Effects of pyrolysis temperature on nitrate-nitrogen (NO3--N) and bromate (BrO3-) adsorption onto date palm biochar.

Biochars (BCs) produced through biomass pyrolysis are highly efficient adsorbents for retaining dissolved cations in soil and water. However, their anionic sorption characteristics are unclear and depend on the feedstock used and the pyrolysis temperature. Herein, the adsorption of nitrate-nitrogen (NO3--N) and bromate (BrO3-) anions onto date palm biochar was evaluated by considering the pyrolysis temperature in relation with the initial concentrations, initial pH, and adsorbent dose. Biochars from date palm rachis were produced at 300 °C (BC300) and 700 °C (BC700). The latter exhibited the highest adsorption efficiency of NO3--N from aqueous solutions at an initial pH of 2, which was 25.8%-44.3% (at an adsorbent dose of 4 g L-1) and 47.0%-73.0% (at an adsorbent dose of 10 g L-1). In contrast, BC300 adsorbed BrO3- with an adsorption efficiency of 19.3%, 14.5%, 14.9%, and 13.6% at initial added concentrations of 5, 10, 15, and 20 µg L-1, respectively. However, BC700 showed zero adsorption for BrO3-. The results also showed that the non-linear models of Langmuir and/or Freundlich for NO3--N and BrO3- adsorption described the adsorption data better in most cases. It could be concluded that high pyrolysis temperature BC are suitable for adsorbing NO3--N, and low pyrolysis temperature BC may be used to adsorb BrO3-.