Influence Of Surface Fractal And Biochar On Co-transport Of Nanoparticles And Contaminants In Sand Porous Media

A thorough understanding of the interaction mechanisms between nanoparticles and granular media under variant physical and chemical conditions is of importance for 1)appropriate use of nanoparticles in natural or engineering environment;2)evaluating their effects on contaminants transporting in aquiferous environment;and 3)identifying their harmness to organisms.Lot of researchers found colloid transport in porous media was influenced by surface roughness,especially under unfavorable conditions.Presently,regular shape used to describe surface roughness is unsatisfactory.Limited information is available to evaluate contaminant transport in biochar-amended soil because batch equilibrium method is often used to investigate the adsoption mechanisium between bicohar and contaminants.Therefore,this study applied fractal theory to describe sand surface roughness and calculated interaction energy between latex nanoparticle and faractal surfaces.In addition,cotransport of silica nanoparticles and acetamiprid in biochar-amended sand porous media was conducted through column transport experiments.The main results are in the following.Firstly,a surface element integration approach coupled with extended DLVO theory was used to examine the interaction energy between different size latex nanoparticle and fractal surfaces at different ionic strengths.The fractal surface constructed via the modified Weierstrass-Mandelbrot function with different combination of fractal dimension D and fractal roughness parameter G to fabricate various nano-topographies.According to the transverse interaction energy maps between latex nanoparticle and two representative fractal surfaces at different separation distances,the magnitude of interaction energy revealed positive correlation with fractal roughness parameter G and generally performed oscillatory attraction-repulsion-attraction behavior.Compared to the 10 nm latex nanoparticle,30 nm latex nanoparticle had larger interaction volume so that the interaction energy between 30 nm latex nanoparticle and fractal surfaces was bigger and distributed more uniformly.The separation distance of the most repulsive energy distribution maps depended on ionic strength and latex nanoparticle size.Especially,at 0.1 nm separation distance,it was surprising to find that there existed critical surface roughness(relying on ionic strength and latex nanoparticle size)that affected the amount of adsorption energy.The longitudinal interaction energy map showed the primary minimum and the repulsive energy barrier between latex nanoparticles and fractal surfaces were weakened in the protrusion place both higher and lower than zero plane while the repulsive energy barrier and the secondary minimum were strengthened in the valley place.Secondly,by studying the repulsive energy barrier,the mean attachment efficiency and interaction energy profiles at representative places,we examined adsorption and desorption of latex nanoparticle.Results showed that the repulsive energy barrier was decreased with decreasing fractal dimension D or increasing roughness parameter G.For a fixed fractal surface,the repulsive energy barrier between nanoparticle and fractal surfaces revealed non-monotonic variation with increasing ionic strength.The gaps(i.e.,the disappearance of repulsive energy barrier)were found to distribute beside the lowest place on fractal surfaces depending on ionic strength,fractal dimension D and fractal roughness parameter G.30 nm latex nanoparticles was released more easily at gaps place in lower ionic strength,whereas the gaps place was more favorable for 30 nm latex nanoparticle adsorption in higher ionic strength.The mean field attachment efficiency was decreased with decreasing fractal dimension D or increasing fractal roughness parameter G.The different discrete scales had negligible effect on the mean field attachment efficiency.In 10 mM ionic strength,there existed "critical nanoparticle size"affecting the mean field attachment efficiency,which was different compared to other ionic strengths that mean field attachment efficiency decreased with increasing nanoparticle size.Pronounced differences between interaction profiles of the protrusion and the pit in the valley place indicated that the protrusion was favorable for particles attachment on smoother surface at higher ionic strength and detachment on rougher surface at lower ionic strength.However,adsorption and desorption of latex nanoparticles were influenced by latex nanoparticle size and ionic strength,and thereby,were more complex on the pit.Thirdly,we conducted saturated column experiments to examine the cotransport of acetamiprid and silica nanoparticles in pure and biochar-amended sand.Retention of acetamiprid was minor in the pure sand,whereas application of biochar in the sand significantly increased retention.Retention was greater at lower ionic strengths and near neutral pH values and was attributed to biodegradation and sorption through ?-? interaction and pore filling.The convection-diffusion equation with inclusion of first-order sorption,desorption,and degradation well described the transport of acetamiprid in the biochar-amended sand.The simulation results showed that the sorption rate did not change with pH.This is because the acetamiprid is nonionic and cannot be bonded with the biochar by protonation or deprotonation.The desorption rate was independent of variation of solution chemistry,indicating that desorption was a physical process(i.e.,pore diffusion).Application of biochar in the sand had little influence on the transport of silica nanoparticles in NaCl but caused complete attachment in CaCl2.Energy dispersive X-ray spectroscopy suggested that the enhanced attachment was due to cation bridging between silica nanoparticles and functional groups in biochar by the Ca2+.The co-presence of acetamiprid and silica nanoparticles in the solutions enhanced transport of acetamiprid and silica nanoparticles in the biochar-amended sand by competing for the binding sites on the biochar surfaces.Our study represents using fractal theory can describe surface roughness more precisely so that interaction energy between nanoparticle and rough surface were more accurate;applying biochar to repair soil,it is critical to consider the co-interaction between nanopartciles and contaminants in different chemical conditions.