Attachment And Transport Of Clay Colloids And Quantum Dot Nanoparticles In Saturated Porous Media

Colloidal particles(i.e.,organic,inorganic and biocolloids)mostly originate from in situ generation of submicron-sized mineral and organic matter which are ubiquitous in the subsurface environmental systems.The colloidal particles may also introduce into the subsurface zone through waste management procedures,such as landfills,septic tanks,or groundwater recharge.These colloids can easily transport through porous media due to their comparatively small size.Colloidal fine materials strongly associated with contaminants which can increase the apparent solubility of low soluble contaminants in subsurface environments.Colloids can also accelerate the transport of cationic and anionic metals through porous and fractured media in the subsurface zone.Colloidal particles sorbed contaminants to its surface and make them mobile.However,colloids can block the porous media and can either facilitate or retard contaminant transport which mainly depends on the prevailing conditions of colloids attachment mechanism.Biochar(BC)as an emerging ameliorant can decrease the bioavailability of contaminants in the subsurface environmental systems with additional benefits of soil fertilization and mitigation of climate change.BC derived from agricultural residues have a strong capability to bind chemical contaminants in water including heavy metals and organic contaminants.In addition,the mobility and toxicity of inorganic,organic,and pathogenic microorganism contaminants is decreased due to biochar application in soil.Conversely,dissolved or colloidal biochar particles may also act as a carrier to facilitate the transport of these same contaminants because biochar has strong sorption capacity of contaminants.Biochar addition to salt affected soil may also balance the deleterious effects of salinity by improving cation exchange capacity(CEC)and leaching of salt besides improving soil physical properties.Biochar has been considered as a very significant tool of environmental management..Biochar can be applied as a green environmental sorbent for the soil and water contaminated with organic/inorganic contaminants.Modern research to understand the fate and transport of colloids(carrying contaminants)in saturated porous media needs to give more preference to predict the potential for colloid-facilitated contaminant transport in the subsurface environment.This Ph.D.research work used Derjaguin-Landau-Verwey-Overbeek(DLVO)theory and different simulation models to systematically investigate and quantify the attachment and transport mechanisms of colloids through column and batch experiments in saturated porous media at different solution ionic strengths.We examine the influence of biochar on deposition and release of clay colloids in saturated porous media through theoretical calculations and laboratory column experiments.Although the potential use of biochar in soil remediation has been recognized,the influence of biochar on the transport of clay colloids,and accordingly the fate of colloid-associated contaminants in the subsurface environment is still unclear.This study systematically conducted three-step saturated column experiments to investigate the transport of clay colloids in biochar-amended sand porous media in various electrolytes at different solution ionic strengths at pH 7.All column transport experiments were carried out at a flow rate of 5 × 10-5m s-1.The obtained breakthrough curves were simulated by implementing HYDRUS 1-D simulation software that numerically solves the Fickian-based convection-diffusion equation(CDE)with non-linear equilibrium and kinetic reactions,which included a first-order deposition and release terms.The deposition mechanisms were interpreted by calculating DLVO interaction energies.A linear relationship between the simulated deposition rate or the attachment efficiency and the fraction of biochar was observed(R2≥0.91),indicating more favourable deposition in biochar than in sand.The DLVO interaction energy calculations showed that the greater deposition of clay colloids in biochar occurs because the half-tube-like cavities on the biochar surfaces favour deposition in secondary minima and the nanoscale physical and chemical heterogeneities on the biochar surfaces increase deposition in primary minima.The deposited clay colloids in NaCl can be released by reduction of solution ionic strength,whereas the presence of a divalent cation(Ca2+)results in irreversible deposition due to the formation of cation bridging between the colloids and biochar surfaces.The deposition and release of clay colloids on or from biochar surfaces not only alter their mobilizations in the soil but also influence the efficiency of biochar for removal of contaminants.Therefore,the influence of biochar on clay colloids transport must be considered before application of the biochar in soil remediation.This study also investigates the effects of solution ionic strengths and particles size on adsorption and desorption of quantum dot(QD)nanoparticles(i.e.,model colloids)in saturated sand porous media through laboratory batch experiments and theoretical modelling.QDs are novel engineered nanoparticles(ENPs)that have varietal environmental and engineering applications.Prediction on the fate and transport of these ENPs in the subsurface environment is very important to understand their mobility.Though numerous column experiments with ENPs including QDs have been conducted to predict the transport of these materials in the subsurface environment but the adsorption and desorption behavior of smaller QD nanoparticles at different solution IS through batch adsorption studies need to be clearly understood.The attachment and detachment behaviour of CdSeS/ZnS alloyed QD nanoparticles were studied systematically in the batch system over a wide range of solution ionic strengths(IS).Water soluable QDs suspension at a concentration of 10 mg L-1 with 6 nm diameter was used as model colloids in the kinetic batch systems.The QD nanoparticles were first deposited in kinetic adsorption batch system using 0.001,0.01,0.1 and 0.2 M NaCl solutions.The concentration of the QD suspensions in the batch system was measured at different time intervals(i.e.,0,5,10,30,60,120 min)to know the attachment behavior of QDs on sand surface.Equilibrium adsorption experiments were also conducted to understand the reversibility of the deposited QD nanoparticles.The removal percentage of QD nanoparticles from batch system increases with increasing solution IS and the highest removal percentage was observed in 0.2 M NaCl solution.The adsorption of QDs on sand surfaces was modelled using Pseudo-first,Pseudo-second order kinetic and Elovich models.The equilibrium adsorption isotherms data were fitted by Langmuir,Freundlich and Temkin isotherm models.Our experimental results revealed that the adsorption of QDs on sand surface increased with increasing solution IS and the kinetic data were found to be the best fit to Pseudo-second order adsorption model at higher IS(e.g.,0.1 and 0.2 M NaCl).The Langmuir adsorption isotherm model was found to be the best fit to the experimental data considering R2>0.99 in all ionic strength conditions.The attachment rate and values of the sand surface area favourable for attachment were varied due to differences in the hydrodynamics of the system,and the role of adsorbent surface roughness.The quartz sand bear nanoscale physical and chemical heterogeneities on their surfaces which increases the deposition of QD nanoparticles in primary minima.Classical DLVO theory predictions were found to be inadequately describing interaction energies between colloids and sand surfaces.The presence of carboxyl functional groups on CdSeS/ZnS alloyed QDs in the solution plays a key role in controlling QDs mobility in the subsurface environment.Our study indicates(i)the influence of biochar on soil process must be thoroughly understood before application of the biochar for soil remediation because biochar has greater affinity for attachment of clay colloids compared with sand particles.The attached clay colloids on the biochar surfaces have both positive and negative effects on removal of a variety of organic and inorganic contaminants in soil.Specifically,the attached clay colloids may mask the active sites on the biochar surfaces for sorption of the contaminants,or the clay colloids may serve as additional collectors for sorption of the contaminants.The clay colloid-associated contaminants will be immobile on the biochar surfaces if the clay colloids are attached in the presence of bivalent cations(e.g.,Ca2+).Conversely,the contaminants will be released with the clay colloids on reduction of solution ionic strength(e,g.,rainfall events)if the clay colloids were initially attached in 1:1 electrolyte(e.g.,NaCl),(ⅱ)The kinetic model confirmed the dominance of chemisorptions mechanism in adsorption of QD nanoparticles on sand We observed that the larger colloidal particles follow kinetic models whereas the smaller colloidal particles including ions follow equilibrium adsorption isotherm model,and(ⅲ)it is important to combine the mechanical(e.g.,reduce collector surface roughness or increase flow rate)and chemical(e.g.,decrease in solution ionic strength)methods for efficient colloidal particles removal,which is mandatory in many industrial and environmental cleaning process(e.g.,cleaning of semiconductor surfaces and remediation of contaminated soils)...