| Resources depletion,non-point source pollution,and soil heavy metal pollution have posed significant threats to food safety,environmental quality and sustainable development of society and economy.Biochar is a solid material derived from the thermal conservation of biomass under no/low oxygen conditions.Owning to the high carbon content,large surface area,developed porous structures,enriched surface functional groups,and existing inorganic compounds,biochar has demonstrated its multifunctional roles in agricultural,carbon sequestration,and pollution remediation.However,less attention was paid to the nutrient properties of biochar and its ability in heavy metal polluted soil remediation.Therefore,in this manuscript,the potential of utilization biochar for nutrient recycling and heavy metal stabilization were evaluated.Firstly,corn straw?CS?,wheat straw?WS?,rapeseed straw?RS?and peanut shell?PS?were selected as the feedstocks,biochars were then produced at different pyrolysis temperatures?300,400,500,600,and 700°C?with different durations?1,2,4 h?.The basic physicochemical properties,water-soluble nutrient contents,and the adsorption capacities of plant nutrients(NO3-,NH4+,and PO43-)on biochars were evaluated in order to understand the nutrient properties of biochars.Secondly,chicken manure was pre-treated with several metal salts in order to analyze the effect of metal salts pre-treatment on biochar nutrient properties and carbon sequestration potential.Carbon loss during biochar production process was also calculated.In addition,biochar abiological/biological stabilities were evaluated chemically and biologically.Thirdly,surface-enhanced Mg salts modified biochars were produced to improve biochar adsorption capacity on PO43-,the efficiency and mechanism of PO43-adsorption on engineered biochars were evaluated.As for heavy metal stabilization,?surface-enhanced?magnesium?Mg?salts modified biochar and the pristine biochar were utilized for Chromium[Cr?VI?]removal from aqueous solution.The effects of different treatments,solution pH,ionic strength,initial Cr?VI?concentration,and reaction time were evaluated,besides desorption and regeneration experiments were conducted to analysis biochar re-generation properties.Moreover,straw?CS,WS,and RS?derived biochars under different temperatures were utilized for heavy metal[Zinc(Zn2+),Cadmium(Cd2+),and Lead(Pb2+)]removal from solution.Biochars were also utilized for heavy metal polluted soil remediation.The effect of biochar addition on soil physio-chemical properties,heavy metal stability as well as its accumulation in plants were evaluated.The main research results were listed as follows:?1?In order to explore the relationship among biochar properties,feedstock,and production conditions,biochars derived from the straw materials under different conditions were characterized for their physicochemical properties,nutrient properties and stabilities.Biochar properties were affected by both feedstock varieties and pyrolysis temperatures.With the rising pyrolysis temperature,biochar pH,ash content,and the degree of aromatization were improved.However,the oxygen?O?and hydrogen?H?content and the number of surface functional groups were decreased.The prolonged pyrolysis duration has the same effect as the rising pyrolysis temperature on biochars.In addition,the effects of prolonged pyrolysis duration were manifest when biochar was produced at a lower temperature<500°C.By contrast,no significant difference was observed among biochars produced at a higher temperature>500°C.The water-soluble nitrogen?N?content accounted limited amount of total N in biochars,and the rates decreased with the rising pyrolysis temperature.Biochar phosphorus?P?content was increased with the rising pyrolysis temperature,while its availability decreased through pyrolysis.Only 4%of the total P in biochars can be extracted with H2O.By comparison,potasium?K?in biochars has high solubility,and 40%of the K in biochars was water soluble.Apart from pyrolysis temperature,the water extractable nutrient contents in biochars were positively related to the nutrient content in feedstocks.Biochar showed a high affinity to NH4+.However,the removal efficiency of NO3-and PO43-were low or even negative for some biochars.As calculated,2.5%?wt%?biochar addition in soil could supply plants with enough K.However,N and P are not surficient,which means other fertilizers are needed in agricultural production.?2?The effects of mineral salts addition on chicken manure carbon loss during pyrolysis,biochar nutrient properties and stabilities were evaluated.Metal salts?especially FeCl3·6H2O?addition can reduce the carbon loss during biochar production.Metal salts addition decreased the P availability in biochars.However,there was no effect of metal salts addition on the availability of other elements such as N,copper?Cu?,and Zn.Biochar abiological/biological stabilities were improved with the introduction of metal salts.The involvement of metal ions in biomass thermal conservation and the physical protection due to the formation of metal oxides on biochar surface were the main reasons for the reduced C loss during biochar production and the enhanced biochar stability.Additionally,the formation of insoluble phosphate would be the main reason for the reduced P availability in biochars derived from mineral salts pre-treated chicken manure.?3?Surface-enhanced MgO modified biochar were produced by impregnating MgCl2·6H2O with sugarcane leaves,which was pretreated with diluted sulfuric acid.The adsorption of PO43-on biochars and the underlying mechanisms were evaluated.Diluted sulfuric acid pretreatment broke the original structure of biomass,increased its surface area,and facilitated the formation and distribution of MgO on biochar surface.Therefore,the surface-enhanced MgO modified biochar showed an improved removal efficiency on PO43-.Precipitation,ionic exchange,and electronic attraction were the main mechanism for PO43-adsorption on biochars.?4?The removal efficiencies of Cr?VI?on biochars under different conditions were evaluated.Solution pH was the main factor that affecting Cr?VI?adsorption on biochars,and the removal efficiency decreased with the rising solution pH.Diluted sulfuric acid pretreatment and MgO modification both enhanced Cr?VI?adsorption on biochars.Chemical interaction between MgO and Cr?VI?was the dominant mechanism for Cr?VI?adsorption on MgO-modified biochars,which is different from that of Cr?VI?adsorption?adsorption-coupled reduction?on the pristine biochar.Most of the adsorbed Cr?74.3%?on MgO-modified biochar existed in the hexavalent oxidation state,whereas it was 33.7%for SHR550.The increased solution pH significantly reduced the Cr?VI?removal on MgO-modified biochars.Furthermore,the Cr?VI?adsorbed on MgO-modified biochars demonstrated a high stability,and MgO-modified biochar had a high reusability potential.This study would provide a reference for designing an optimal biochar for the effective removal of Cr?VI?from wastewater.?5?The effects of biochar addition on soil heavy stabilization were evaluated.Biochars derived from different feedstocks at different pyrolysis temperatures were utilized for heavy metal(Zn2+,Cd2+,and Pb2+)removal from aqueous solution as well as stabilization in polluted soils.Biochars produced at higher temperatures have higher adsorption capacity for heavy metals.As for the contaminated soils,biochar addition increased soil pH and the organic matter content.Heavy metal availabilities were reduced through stabilization.Metals were transformed from the acid extractable fraction into the oxidable fraction.Phytotoxicity test revealed that biochar addition reduced heavy metal accumulation in ryegrass roots.In addition,the effects of biochar addition on soil heavy metal stabilization were also related to the metal contents in soils as well as their specification.All in all,biochar production showed great potential for plant nutrient?especially P and K?recycling and soil heavy metal stabilization.The obtained results can provide theoretical basis for biochar agricultural utilization and heavy metal polluted soil remediation. |
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