The Environmental Applications And Corresponding Mechanisms Of Biochar Derived From Aquatic Eco-Remediation Plants

Biochar is a carbonaceous solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment,which has received much attention for climate change mitigation,soil improvement and environmental remediation.Currently it remains challenging to clarify some critical issues related to the environmental application of biochar,such as the quantitative study on mechanism of cadmium sorption on biochar,simultaneous sorption and catalytic oxidation of trivalent antimony by biochar,and synthesis of biochar based granular sorbent for phosphorous removal.In this study,biochars were prepared using aquatic plants at different pyrolysis temperature and characterized by elemental analysis,Zeta potential analysis,FTIR,TGA,SEM-EDS,BET,XRD,XPS,and EPR,and their sorption capacity and mechanisms for NH4+,Cd2+,PO43-,and Sb(III)were particular determined.Considering the important role of wetland system in climate change,the carbon sequestration potential of wetland plant-derived biochars was also explored in the present research.These results promote the reclamation of wetland plants and give us a strong reference with regard to the industrialized development of constructed wetland,and provided a theoretical basis for the synthesis and application of functional biochar.The main original conclusion of this study are drawn as follows:(1)We systematically determined the physicochemical properties and structure of biochars derived from 22 species of aquatic plant,and found that they have great potential as amendments for acidic soils due to the high pH and CEC values.The biochars had a R50 recalcitrance index of 0.42-0.50 and the carbon sequestration potential of 19.4-28%.The aquatic plant biomass from all the Chinese constructed wetlands if made into biochars has the potential to sequester 11.48 Mt carbon yr-1 in soils over long time periods,which could offset 0.4%of annual CO2 emissions from fossil fuel combustion in China.(2)In comparison to the agricultural residues derived biochar,the wetland-plant derived biochar showed a considerable sorption capacity towards ammonium and cadmium,especially for Canna indica derived biochar.The sorption kinetic data could be well described by a pseudo second order model,and sorption of NH4+ on biochars is more quickly to attain equilibrium than Cd2+.Langmuir model fits the isotherm data better,and the biochars showed a higher affinity for Cd2+.Inorganic precipitation,ion exchange,coordination with ? electrons,and complexation with surface oxygen-containing functional groups were the general mechanisms of Cd2+ and NH4+ sorption on the biochars,and the specific mechanisms were greatly affected by the property of sorbate.(3)We qualitatively and quantitatively determined the mechanisms of cadmium sorption on biochar,and precipitation with minerals,ion exchange,complexation with surface oxygen-containing functional groups,and coordination with n electrons were the possible mechanisms of Cd2+ sorption on the biochars.With increasing pyrolysis temperature,the effect of precipitation and Cd2+-? interaction on Cd2+ sorption significantly increased,while the effect of surface complexation and metal ion exchange decreased.As the pyrolysis temperature increased,the contribution of organic components to Cd2+ sorption decreased from 27.0%to 5.8%,while the contribution of the minerals significantly increased from 73.0%to 94.2%.In general,the interaction with minerals,precipitation with the minerals and metal ion exchange,dominated the Cd2+sorption on the biochars.(4)We successfully synthesized magnesium-alginate modified biochar microspheres to enhance removal of phosphate from aqueous solution and overcome the drawback of lower collection and regeneration efficiency of powder biochar.The modified microsphere exhibited greater phosphate removal rate than original sample,and precipitation with minerals and ligand exchange were the possible mechanisms of phosphate sorption.Notably,it also showed considerable phosphate removal efficiency(?90.0%)when applied to eutrophic river water and wastewater from wastewater treatment plant and pig farm,indicating MgCl2-alginate modified biochar microspheres have potential as a green cost-effective sorbent for remediating P contaminated water environment,which provided a theoretical basis for the synthesis and application of functional biochar.(5)We revealed the simultaneous sorption and catalytic oxidation of trivalent antimony by biochar,and a two-step Sb(III)oxidation reaction were involved in the sorption process.In the first stage,molecular oxygen was reduced by electron donating moieties in biochars and converted into some reactive oxygen species,and the Sb(?)in aqueous solutions was then oxidized to Sb(?)by the generated reactive oxygen species in the following step.Additionally,the oxidation of Sb(?)occurred at the biochar surface is the major pathway,together with the transformation taking place directly in aqueous solution.The results from the present study demonstrate that Canna indica derived biochars can effectively remove Sb(?)from contaminated water,and simultaneously reduce the toxicity of Sb by catalyzing Sb(?)oxidation into Sb(?).These insights evoke the prospect of biochar as a catalyst with electroactivity,and will give us a strong reference with regard to Sb(?)-enriched wastewater disposal.