Phytoremediation Potential of Maize (Zea mays L.) using Biochars Produced from Walnut Leaves in a Contaminated Soil

  TPTEs pollution can be hazardous to soil, plant, and human health through the soil-crop-food chain. Lead (Pb) is a toxic element commonly found in heavy-metal contaminated soils, and it has been one of the major global environmental concerns over the past few decades. It discharges to the soil environment through increased anthropogenic activities especially mining. This metal can have harmful and chronic-persistent health effects on exposed populations through food consumption grown on contaminated soils. It is needed to reduce bioavailability of metals in croplands. Biochar is an organic C rich material derived from pyrolysis of waste biomass under an oxygen-limited environment. It is used in soil to immobilize soil PTEs, The aim of this study was to evaluate the feasibility of Walnut leaf (WL) and its derived biochars (WLB) produced at three pyrolysis temperatures of 200, 400 and 600 °C on lead (Pb) availability and its accumulation in a cultivar of maize (Zea mays L. Cv. Single cross 704) grown in a contaminated calcareous soil. An experiment was conducted with maize grown in untreated soil (control) and soil treated with three rates of WL and WLB applications. Dry amendments were mixed with a mass fraction of 0, 0.5, 1, and 2% (w/w). Plastic pots were filled with 3 kg of amended and unamended soil. Each treatment was performed in triplicate. The pots were incubated for the soil mixture to equilibrate over 45 days. After the incubation period, macro- and micro-nutrients were added to all treatments according to the soil test. In each pot, 3 seeds of maize were sown and plants were grown for 8 weeks. They were harvested and separated into roots and shoots. The dried shoots and roots were grounded and stored for further analysis (maize indices: shoots and roots dry weight, Pb concentration in shoots and roots, bioaccumulation factor and translocation factor). Soil samples from pots were analyzed too (DTPA-extractable and total Pb). This study has demonstrated that the application of the amendments to the calcareous contaminated soil has the potential to reduce the phytoavailability (bioavailability, bioaccumulation factor) of Pb to maize. Influence of amendments on Pb availability and uptake varied depending on the pyrolysis temperature and application rate. Phytoavailability of Pb was most dramatically influenced by biochars addition. The 0.5, 1 and 2% (w/w) biochar prepared at 600 °C, significantly decreased Pb concentration in the shoot by 31.3, 33.5, and 36.1% respectively and in the root by 32.0, 35.6, and 36.2% respectively (p<0.05). Amendments reduced Pb uptake in the shoot/root of maize too. Physiological responses showed that amendments application improved the shoot/root growth and dry biomass (root and shoot). In comparison with the control, the highest shoot and root dry matter values were determined in 2% (w/w) biochar-600 °C treatment by 131.4 and 116.7% respectively (p<0.05). Correspondingly, the bioaccumulation factor of Pb also decreased with increasing amendments pyrolysis temperature and addition rate. Results indicated that the DTPA-TEA-extractable Pb was significantly (P< 0.05) reduced in soils treated with amendments. Bioavailable soil Pb concentrations (DTPA extraction) decreased by increasing amendments rate and pyrolysis temperature. The 0.5, 1 and 2% WLB produced at 600 °C, significantly decreased the DTPA-extractable Pb in comparison with the Control by 35.3, 40.1 and 49.1%, respectively. Therefore, these results indicated that amendments inhibited the uptake and transfer of Pb by maize plants. This study clearly has shown that biochar has the potential of immobilizing Pb, reducing its availability to maize, and increasing plant growth. Thereby biochar can reduce lead exposure and increase its phytostabilization, associated with phytoremediation potential of maize