Study On The Mechanisms Of Arsenic Sorption And Stabilization In Soils Using Ferrihydrite Nanoparticles

Arsenic（As）is one of the toxic elements with contents exceeding the soil and groundwater limits in some areas of China.The accumulation of arsenic（As）in soil,particularly inorganic As,is an important issue because of the adverse effects it may have on food quality,human health,and the environment.As a means of countering these negative effects,in situ immobilization of As at contaminated sites,by the addition of ferrihydrite（HFO）,has been proposed as an attractive and low-cost remediation alternative.However,the poorly crystalline HFO transforms into more crystalline iron forms,such as hematite or goethite,over time,greatly diminishing its high reactivity with As and efficiency in its removal.Nanoscale particles have larger specific surface area and potentially higher reactivity than bulk particles or natural minerals,which should confer greater sorption capacity.In the study,HFO nanoparticles modified with carboxymethyl cellulose（CMC）as a stabilizer was synthesized and used to enhance As removal efficiency in water and soils.Firstly,As sorption kinetics and isotherm on HFO nanoparticles in solution were carried out to elucidate the immobilization mechanism following the surface complexation model（SCM）and FTIR,XPS,XRD analyses;Then,to evaluate the stabilization of As on HFO nanoparticles following soil incubation,meanwhile analyze the As fractionation in soils;We conducted a pot study to examine soil As availability and uptake by rape（Brassica campestris L.）during two harvest periods in response to amendment with HFO nanoparticles for As-contaminated soil;Finally,As immobilization using HFO nanoparticles was investigated through a series batch and soil column experiments to examine the mobility of the stabilized HFO nanoparticles in soils and their co-transport behaviors with As.Observations will provide theoretical references for in situ remediation As-polluted soils and the safety use of farmland.The main results are shown as follows:1.CMC might be the optimum stabilizer for HFO nanoparticles because of more effective physical and chemical stability.The large-pore structure,high surface specific area,and the non-aggregated nature of CMC-HFO lead to increased adsorption sites,and thus high adsorption capacities of As（V）without pre-treatment(355 mg·g^-1),which is much greater than those reported in previous studies.2.Second-order equation fit well to the sorption kinetics of As on HFO nanoparticles,which suggests that rate-limiting step is chemical adsorption rather than diffusion.Dual-mode isotherm model could be successfully used to interpret the isotherms of As（V）.The results illustrate precipitation becomes more important following the increased As concentrations.At lower concentration of As（V）(Ce<9.39 mg·L^-1),more than 97%of the observed As（V）removal was attributed to adsorption.Increased As（V）loading saturated the HFO nanoparticles’adsorption capacity,giving rise to precipitation:at Ce>40 mg·L^-1,15%-28%of As（V）removal was due to precipitation.3.FTIR,XPS and XRD analyses suggested that precipitation and surface complexation were primary mechanisms for As（V）removal by HFO nanoparticles.Solution pH can affect both arsenate speciation and surface charge of HFO nanoparticles and further change the sorption capacity As（V）.A surface complexation model（SCM）was used to simulate As adsorption over pH 2.5–10.4.The predominant adsorbed arsenate species were modeled as bidentate binuclear surface complexes at low pH and as monodentate complexes at high pH.4.The adsorption experiments were performed in wastewater samples from Realgar mine tailings with an initial arsenic concentration of 38.2 mg·L^-1,since the composition of real wastewater is more complex.Although large amounts of other species exist in the wastewater,modified HFO nanoparticles still show an excellent performance with an adsorption efficiency of⁹0.5%for As（V）.For continuous treatment by HFO nanoparticles for three times,the concentration can be decreased to 5.6μg·L^-1,far below the standard of 10μg·L^-1 suggested by World Health Organization（WHO）.5.As stabilization efficiency decreased to 70.9%and As TCLP concentration was down to 17.9μg·L^-11 when HFO nanoparticles as a passivator.For bare HFO particles,the stabilization of arsenic in soil was mainly attributed that As was transformed from non-specific adsorption and the specific adsorption state into the crystalline Fe-Al hydrated oxides state.While As fraction in F1 and F2 was transformed to amorphous crystalline Fe/Al oxide-sorbed As for HFO nanoparticles.6.Application of modified HFO nanoparticles in soils not only provided a larger specific surface area,but also markedly improved stability against aggregation and recrystallization.For 90 d incubation,bare HFO particles were gradually converted to the crystalline Fe（III）oxide form（76.6%）,although this was not observed for the 0.5%HFO nanoparticles（only 2.8%）.7.The ratio of amorphous and free Fe-oxides（Fe_o/Fe_d）showed significant positively correlated with solid-solution distribution（K_d）and amorphous crystalline Fe/Al oxide-sorbed As,which suggests that a larger amount of As is associated with Fe（hydr）oxide in the amorphous phase or smaller particles.8.CMC-stabilized HFO nanoparticles could be more effective in lowering the As uptake by rape and available As in soils than bare HFO particles.Compared with bare HFO particles,the addition of0.5%modified HFO nanoparticles to soils reduced the phytoavailability of As in edible rape by 30.5%and 61.0%at the first and second harvest,respectively,and available As in soils decreased by 40.1%and 52.9%,respectively.However,not all treatments induced an increase in rape biomass.The results concluded that prolonging the incubation period tended to increase immobilization of As,thereby indicating that longer periods of time are needed to remediate polluted soil and promote plant growth when amending soil with low concentration of HFO particles.9.Column tests proved that CMC stabilized HFO nanoparticles were highly mobile in the soil under a pressure.Batch and columns experimental results revealed that the nanoparticles treatment greatly reduced water-leachable As from the soil.The water leachable As from As polluted soil was reduced by 67.1%and 78.7%for CZ and SM soils,respectively.The total As breakthrough curves of soils suggested that As and the stabilized nanoparticles have a co-transport behavior in soil,and it may be related to soil properties and soil water velocity.