Examination of Three Different Organic Waste Biochars as Soil Amendment for Metal-Contaminated Agricultural Soils

Abstract The efficiency of biochars (BCs) derived from paper mill sludge (PM-BC), distillery sludge (DS-BC), and pruned branches from roadside trees (PB-BC) for immobilization of selected heavy metals (Cd and Zn) in agricultural soils was examined in the current study. This examination was conducted in order to elucidate whether the recycling of organic wastes could be used as a management option for metal-contaminated agricultural soils. Biochars were applied to contaminated upland soil exceeding the guideline values for Cd (4 mg kg^-1) and Zn (300 mg kg^-1) in Korea. Both the incubation study and the pot trial with lettuce cultivation were carried out using soil-BCs mixtures at 0, 1, 2, and 5 % (w/w). From the incubation study, it appears that BCs incorporated into the soil induces an increasing soil pH and a significant decline (Cd 36 ∼ 100 %; Zn 54 ∼ 100 %) in the phytoavailable metal pool examined by 1 M NH₄NO₃ extraction. The PM-BC was most effective in the reduction of Cd and Zn phytoavailability, due to a significantly higher pH and surface area than the DS-BC and PB-BC. Similar results were observed in the pot trial, where the uptake of heavy metals by lettuce greatly declined with PM-BC incorporation (Cd 26 ∼ 71 %; Zn 28 ∼ 45 %). PM-BC enhanced the lettuce growth performance evidenced by the highest yield of lettuce being observed with PM-BC-treated soils at 5 %. This was attributed to retardation of the metal toxic effect induced by a decrease in Cd accumulation, while the increased nutrient elements originated from PM-BC. This present study indicates that paper mill sludge is a great candidate for biochar production that can be utilized as a soil amendment for metal-contaminated agricultural soils.