Physicochemical Property And Environmental Stability Of Micron-and Nano-particle Biochars

Biochar as an effective multifunctional material in soil remediation,soil improvement,and carbon sequestration has attracted wide interest recently.However,the ever-increasing land application of biochar may raise some potential environmental risks.Biochar may influence the soil structures and chemical/biological behaviors of natural substances in soil,and may even affect the transport and transformation of contaminants.Especially,biochars with small particle sizes such as micronparticle(MP)and nanoparticle(NP)biochars have high mobility which could act as carriers to facilitate the transport of contaminants in soil.Feedstock largely influence the structure and property of biochars,and would further influence the composition and environmental stability of MP and NP biochars.In this study,a variety of biochars were produced from pyrolysis of nine biomasses at 500?and then subjected to the extraction of MP and NP biochars.The diverse physicochemical properties of MP and NP were characterized by multiple methods of XRD,SEM,TEM,etc,and Time-Resolved Dynamic Light Scattering was applied for colloidal stability determination.The main results were as follows:(1)Biochars contained a considerable amount of MP and NP derived from different biomass sources.The yields of MP biochars with particle diameters smaller than 1?m were 1.43-20.5%,and the yields of NP biochars with particle diameters smaller than 100 nm were 0.99-15.3%;among them,herb biomass derived biochar had highest MP and NP proportions.The MP and NP biochars contained more O-containing functional groups and mineral components but less aromatic clusters than the bulk biochar.Plant-derived MP and NP biochars from natural biomasses contained more condensed aromatic rings and functional groups than those from dairy manure and sewage sludge.However,the latter MP and NP biochars were rich with minerals such as carbonates,phosphates and silicates due to the input from anthropogenic activities.(2)The stability of biochars were largely influenced by particle sizes and biomass sources.MP biochar had higher zeta potential than NP biochar,indicating higher stability.Plant-derived biochars were always more stable than waste biochars.Wheat straw MP had highest zeta potential value and could remain stabilized in increasing Na~+solution while sewage sludge and dairy manure MPs were readily destabilized,with the Critical Coagulation Concentration(CCC)of 200 mM and 75 mM,respectively.The similar phenomenon was observed for NP,but with about 25 mM higher CCC.The different stability of MP and NP biochars could be attributed to the surface O-containing functional groups and mineral compositions.(3)Clays in soil influenced the aggregation behavior of biochar colloids.Negatively charged kaolinite enhanced the stability of MP and NP biochar colloids due to the electrostatic repulsion.Biochar and kaolinite colloids binary system had higher CCC value than biochar or kaolinite alone,therefore the stability against ion strength was enhanced after kaolinite and biohar mixed.Increased zeta potentials in mixed system accounted for the high electrostatic repulsion,which was in line with DLVO theory.Besides,biomass sources also influenced the heteroaggregation of mixed system.(4)The heteroaggregation between biochar colloids and metal metal oxides was largely influenced by their ratios.Biochar colloid mixed with positively charged goethite colloids experienced destabilization and redispersion.Functional groups of biochar colloids interacted with goethite which enhanced the aggregation by reducing the net charge of mixed system;with the concentration of biochar colloids further increasing,the complex of biochar colloids and goethite became high negatively charged which make the system become restable.Biomass sources also influenced the heteroaggregation process,wheat straw and dairy manure biochars had higher ion contents which hinder the formation of the restabled system.Overall,this work revealed the size-dependent characteristics of compositions and structures as well as high colloidal stability of MP and NP which are helpful for the prediction of their environmental fate and risk.