Biochar Mediated Lead And Copper Stabilization In Naturally Contaminated Paddy Soil:Mobility And Phytoavailablity

Contamination of agricultural lands with Pb and Cu are extremely hazardous for the environment and safe food production,and plentiful efforts have been put forward for their remediation so far.Biochar is an emerging organic and eco-friendly amendment for remediating the metal contaminated soils.Rapeseed residue(straw,leaves,and pods)in comparison with rice straw biochars pyrolyzed at different temperatures for Pb and Cu immobilization has not been reported earlier.A set of incubation and greenhouse pot experiments were conducted to assess the influence of the applied biochars on the Pb and Cu immobilization,phytotoxicity attenuation,soil biochemical properties,redistribution under various moisture levels,and residual effects on Pb and Cu stabilization in naturally contaminated paddy soil.The main results showed:(1)Rice straw biochar prepared at high temperature(RS550)had high pH of 10.52,followed by 9.98 in rapeseed residue biochar at high temperature(RP550),while low-temperature biochars had low pH.The negative charge on biochar particles increased with pH,which was consistent with the decreased zeta potential(increased negativity)values.Variations were observed in FT-IR,SEM,and XRD analysis,with silicon oxide only found in rice straw biochars.Total acidity was decreased in biochar made at high temperature,while total surface basicity increased at increased temperature.The total and bioavailable Pb and Cu contents were decreased for high-temperature biochars,making them suitable for land use.(2)The efficacy of biochars on Pb and Cu immobilization were evaluated by Community Bureau of Reference(BCR)sequential extraction,toxicity characteristics leaching procedure(TCLP),and single extraction(CaCl2)in a greenhouse experiment.The results revealed that the mobile and relatively toxic fractions of Pb and Cu were transformed into most stable(residual)fractions.Substantial reduction in Pb and Cu phytoavailability and solubility were observed following the addition of biochars.Maximum percent reduction in Pb concentration in roots and shoots recorded was 37.81%and 26.54%respectively,whereas root and shoot concentration of Cu was reduced by 50.41%and 46.25%respectively.Enhanced soil urease,catalase,and acid phosphatase activities after BCs application reflected the improvement of soil biochemical properties.(3)The relative efficiency of rapeseed residue and rice straw biochars on transformation and reduction in bioavailability of Pb and Cu was compared in an incubation experiment.Amending the soil with RS550 significantly decreased the acid-extractable portion of Pb and Cu by 63.30%,and 66%respectively.Whereas the residual(stable)fractions of Pb and Cu was increased by 40.31%,and 52.98%respectively.The immobilized metals were mainly transformed to the reducible fractions.High reduction in the bioavailable phase of Pb(97.13%),and Cu(93.71%)was recorded.The affinity of BCs towards Pb and Cu was enhanced due to the increased negativity of BCs with pH,as described by zeta potential and CEC.Accordingly,this was one of the possible mechanisms of Pb and Cu immobilization in amended soils.(4)Influence of specific moisture levels on redistribution and phytoavailability of Pb and Cu by rapeseed residue and rice straw biochars was systematically investigated in a greenhouse environment.Maximum Pb and Cu activity was found under 80%FC,followed by 60%FC,whereas the activity of these metals varied slightly in 40%FC over control.A prominent reduction in exchangeable fractions of Pb and Cu by 75.42%and 59.46%was recorded in the 5%of RS550 treatment under 80%FC.Likewise,5%RS550 treatment increased the residual fractions of Pb and Cu under high moisture level by 54.76%,and 27.44%respectively.Phytoavailability of Pb and Cu under 80%FC was significantly reduced in the soil amended with 5%RS550(67.66%and 46.50%)and 5%RP550(60.52%and 46.50%)respectively.Reduced availability as noticed in sequential extraction and CaCl2 extraction under increased moisture level led to the reduced Pb and Cu uptake by 44.89%,and 51.26%in R5%RS550,followed by 5%RP550.The active metal redistribution and transformation to more stable fractions at 80%was possibly due to high pH,metals precipitation with sulfides,and increased concentration of amorphous Fe oxide content.(5)The residual impacts of rapeseed residue and rice straw biochars on Pb and Cu immobilization and ecological restoration of contaminated soil were investigated by changes in metal fractionation and phytoavailability over two years.Soil pH mostly sustained over two years.The reduction capability of Pb increased from 53.50%-57.56%,while the percentage of Cu reduction ranged from 50.35%-54.18 in the acid soluble fraction.The residual fraction of Pb increased from 36.17%to 43.85%while,Cu increased from 50.94%-63.47%.The reduction efficiency of bioavailable Pb increased from 60.64%to 75.85%,while the increase in percent reduction of Cu shifted from 57.61%to 70.24%to 5%RS550 respectively.Pb and Cu concentrations in plants were significantly decreased by 40.81%and 56.14%respectively,with no noticeable differences in plant dry biomass over two years.Due to their stability,rice straw and rapeseed residue biochars produced at high temperature were more effective in reducing bioavailability and solubility as compared to low-temperature biochars.The varying efficiencies of biochars to stabilize Pb and Cu were possibly due to the biochar composition and degree of aging.These results exhibited positive impacts on Pb and Cu immobilization by biochars,while soil biochemical properties were improved significantly in RS550 and RP550 respectively.Application of biochars could immobilize Pb and Cu under different levels of moisture,consequently decreasing their uptake by plants.Residual influence of the applied biochars assisted in the ecological restoration of contaminated soils.When compared to Cu,the applied biochars efficiently immobilized soil Pb in the current study.Biochars used in the current study had low contamination risk and therefore would prove advantageous for immobilizing Pb and Cu in contaminated soil.Rice straw biochars synthesized at high temperature as compared to rapeseed residue biochars due to their favorable immobilization properties such as increased pH,CEC,negative charge,mineral composition,and increased alkalinity made them promising stabilization amendments.