Study On Transport Regularity Of Biochar Colloids In Porous Media

Biochar is a stable solid product rich in carbon produced by high-temperature pyrolysis of organic matter(e.g.,agricultural waste,dead branches,and leaves)under hypoxic conditions.It is widely used for improving soil quality,increasing crop yields and the remediation of contaminated sites due to its superior physicochemical properties,good stability,and carbon fixation capability.Once biochar is applied to environment,it undergoes physical,chemical,and biological decomposition,forming small-sized biochar colloids(BCs).These biochar colloids can transport through soil and even groundwater,significantly impacting the global carbon cycle.On the other hand,biochar colloids have a stronger adsorption and transport capacity.It can serve as a vehicle for the transport of pollutants,thus posing potential environmental risks.Therefore,it has important implication for studying the transport regularity on BCs in porous media and the transport mechanism of pollutants to BCs.In this study,BCs prepared from corn stalks were used as the research subject.Firstly,the effects of different environmental factors(p H,ionic strength(IS),and media particle size)on the transport of BCs in one-dimensional sand columns were studied.Secondly,a real time visualization system was used to explore the transport mechanism of BCs in a two-dimensional sand tank.Moreover,the transport behavior of BCs was fitted using the double-Monod model.Finally,phenanthrene was chosen to investigate its impact and action mechanism on the transport of BCs in both one-dimensional sand columns and two-dimensional sand tanks.The research results are as follows:(1)The results of one-dimensional sand column experiment demonstrate that BCs have a strong transport capacity in one-dimensional sand columns.Moreover,high p H value,low IS and coarse media particle size will promote the transport of BCs.Zeta potential absolute value of BCs and quartz sand increases with the increase of p H and decrease of IS,and the electrostatic repulsion between them then increases,resulting in the reduction of the adhesion of BCs on the sand surface,which promotes the transport of BCs.Additionally,the media particle size significantly impacts the transport of BCs.With the increase in media particle size,the pores between sand particles expand,the permeability coefficient rises,and the transportation of in BCs is enhanced.(2)The results of a two-dimensional sand tank experiment demonstrate that the effects of various factors,including p H values,IS,and media particle size,on the transport of BCs in a one-dimensional sand column are consistent with the findings observed in the two-dimensional sand tank.In addition,the Double Monod model accurately fits the transport behavior of BCs(R~2>0.94).Furthermore,with IS increases from 1 mmol/L to 50 mmol/L,the fitted first-order adsorption coefficient(k)value increases from 0.0028 to 0.0046.When the p H value decreases from 10 to 4,the k value increases from 0.0014 to 0.0034,indicating an increase deposition of BCs in the two-dimensional sand tank.Moreover,with the media particle size increases,the fitted k value decreases from 0.0047 to 0.0023,consequently promoting the transport of biochar colloids in the two-dimensional sand tank.(3)The presence of organic pollutant phenanthrene slightly impact on the transport of BCs.As the p H values,IS,and medium particle size conditions vary,BCs display a considerable specific surface area,a wealth of oxygen-containing functional groups,and intricate porous structures.Phenanthrene adsorption on the BCs surface effectively shields its surface charge,consequently reduced transport rate of BCs in porous media.Furthermore,under acidic and high IS conditions,the aggregation tendency of BCs and adsorption capacity for phenanthrene are enhanced.Additionally,under conditions of fine media particle sizes,the contact area between BCs and phenanthrene increases,thereby exerting a significantly inhibitory effect on the transport of BCs.In conclusion,the results of this study contribute to a deeper understanding of BCs transport in complex porous media and the co-transport patterns of associated pollutants.This study will explore the potential of BCs as carriers of organic pollutants and the application of biochar in phenanthrene-contaminated soils,as well as predict BCs environmental fate.