| Biochar(BC),which has been proposed as a promising material for the remediation of contaminated soil,can effectively adsorb,reduce,and immobilize Cr(Ⅵ) and many other pollutants.However,colloidal BC particles generated during the production and application processes can still mobilize and transport in soil and other porous media.After Cr(Ⅵ) adsorption,colloidal BC particles with adsorbed Cr species(hereafter named as Cr-BC) could migrate along the soil profile and pose risk to deep subsurface and underground water environments.Therefore,the influences of Cr(Ⅵ) adsorption and reduction on BC colloid properties and transport behavior in saturated porous media deserve investigation.Biochar samples were prepared via pyrolysis of wheat straw at 450 °C and 600 °C under limited oxygen atmosphere(hereafter named as BC450 and BC600,respectively).The corresponding colloidal BC particles,which were obtained by ball milling,flotation,and centrifugation,were used for batch Cr(Ⅵ) adsorption experiments.The effects of pyrolysis temperature,Cr(Ⅵ) adsorption amount,and ionic strength on BC colloid properties(including surface distribution of Cr species,particle size,and electrokinetic potential etc.) were investigated.The analysis of X-ray photoelectron spectroscopy indicated that over 80% of the total Cr species on Cr-BC surface existed as Cr(III) after 6 h of adsorption reaction.It was also found that the C–O bond of phenol,alcohol and ether moieties on BC surface possibly participated in the reduction of adsorbed Cr(Ⅵ) to Cr(III) species and was oxidized to C=O moieties.Light scattering analysis indicated that the reductive adsorption of Cr(Ⅵ) resulted in less negative zeta potentials and enlarged hydrodynamic diameter of the BC colloids.These could be attributed to the formation of the positively charged Cr(III) species,as well as the loss of the negatively charged oxygen-containing groups on BC surface.Furthermore,the complexation,coprecipitation,and electrostatic attraction with Cr(III) could facilitate the aggregation of Cr-BC,inducing the formation of larger particle sizes.Based on the above results,the effects of Cr(Ⅵ) adsorption amount,ionic strength,and pyrolysis temperature on the transport behavior of BC colloids in saturated quartz sand media were examined.Breakthrough curves and retention profiles of Cr-BC during transport were inversely fitted by a two-site kinetic retention model.It was found that for Cr-BC450 which was formed during reductive Cr(Ⅵ) adsorption onto BC colloid particles,the mobility decreased with increasing adsorption time (0~20 h) and adsorption amount (0~41.5 mg/g).When the ionic strength of the background solution was increased from 1 to 5 mM NaCl,a transition from blocking to ripening occurred in the breakthrough curves of Cr-BC450,the mobility of which decreased.The depth-dependent deposition coefficient significantly increased with higher Cr(Ⅵ) adsorption amount.The calculation based on the Derjaguin-Landau-Verwey-Overbeek(DLVO) theory revealed that Cr-BC particles had less mobility than pristine BC colloids because of decreased energy barrier and deepened secondary energy minimum between particles and collectors.In comparison to BC600,BC450 possessed higher reductive Cr(Ⅵ) adsorption capacity,but demonstrated greater motility as Cr species carriers.The transport of Cr-BC600 was mainly dominated by straining effect. |
PDF Full Text Request