Phytoremediation and effect of soil amendments on chromium and nickel uptake in lowland rice soils affected by mining activities

The study was conducted to evaluate various remediation techniques to reduce the absorption and translocation of Cr and Ni in lowland rice in Barangay [village] Lomboy, Santa Cruz Zambales [Philippines] and at the same time increase rice productivity in the area. The use of vetiver significantly reduced the amount of total Cr by 4.3%, available Cr by 8.53%, total Ni by 9.86% and available Ni by 14.26% in the soil. When grown for 3 months, the roots were able to accumulate 3.83 kg Cr ha' and 3.66 kg Ni ha' with only 0.03 kg Cr ha' and 0.07 kg Ni ha' in shoots. After 8 months, the amount of Cr accumulated in the roots and shoots increased to 6.36 and 0.04 kg Cr ha' respectively. For Ni, it only increased to 4.12 and 3.66 kg Ni ha'. Measured in terms of bioaccumulation factor (BAF), vetiver had a BAF of 0.97 for Cr and 1.07 for Ni after 3 months of growth and 1.48 for Cr and 1.38 for Ni after 8 months, indicating higher concentration of Cr and Ni in the plant than in the soil. The translocation factors at both 3 and 8 months were below 0.2 mg kg * which is the prescribed limit given by the Codex Standard and European Union, suggesting that vetiver accumulates Cr and Ni in the roots and very small amount of Cr and Ni are translocated to the shoots, making the plant safe for forage purposes. Aside from the beneficial effects in mitigating soil pollutants, vetiver also improved soil pH and percent organic matter. The Cr and Ni content in rice grains after planted the area with 3 months vetiver, were efficiently reduced to below detection limits. Also, growing vetiver before rice reduced grain yield since vetiver is also a good accumulator of nutrients. The use of vermicast, carbonized rice hull, bagasse ash and zeolite combined with inorganic fertilizer increased grain yield and at the same time reduced the uptake of Cr and Ni in grains. Total and available Cr and Ni in the soil were markedly reduced as compared to the control. Higher yield was obtained with the addition of vermicast. Zeolite was the most effective soil amendment in mitigating total and available Cr, available Ni while carbonized rice hull for total Ni in the soil. Highest return on investment was realized with the application of bagasse ash due to its cheaper/free cost while lowest ROI was observed with zeolite due to its relatively higher cost.