Effects Of Different Layered Double Hydroxide Materials On Removal And Migration Of Chromium In Sand Filtration System

Chromium is a versatile element widely used in leather tanning,textile printing and dyeing,electroplating and other industries.Chromium has special chemical properties and toxicity,among which Cr（Ⅵ）in form of Cr O₄^2-is a priority contaminant due to its carcinogenicity and teratogenicity,as well as its high solubility and high mobility in environmental media.Mining and industrial activities in the chromium-related industries have produced a large amount of solid and liquid chromium-containing wastes,and their improper storage and disposal has caused significant amount of chromium released into the environment causing the increasingly serious chromium pollution in soil and water bodies,which has seriously affected agricultural safety and human health.Since 2005,China has been taking effective measures to eliminate chromium slag pollutions caused by chromium industries in accordance with the"Comprehensive Treatment Plan on Chromium Waste Contamination".During the chromium contaminated site remediation processes,the remediation of contaminated soils usually adopts wet detoxification techniques,including in-situ and ex-situ soil washing.A large amount of chromium-containing wastewater will be generated during the soil washing remediation.The eluents produced by in-situ and ex-situ ex-situ soil washing need to be effectively treated before being discharged into the municipal wastewater system.Due to the complex composition of soil,the soil washing eluent usually carries large amount of soil colloidal components,including both soil organic matter and soil inorganic minerals,and the latter includes the common iron and manganese oxides and aluminosilicate minerals.The soil inorganic minerals have small particle size,special size effect,and large specific surface area,which may have serious impacts on common chromium-containing wastewater treatment processes.Previous studies have found that layered double metal hydroxide minerals（for example,hydrotalcite Mg/Al LDH and hydrotalcite-like Zn/Al LDH or Fe/Al LDH）formed by divalent metal cations and aluminum-containing minerals,which are important active colloid components in soil.LDH carries a permanent positive charge in the structure showing a strong adsorption capacity for Cr（Ⅵ）oxyanions,and the iron-containing LDH（Fe/Al LDH）is capable of reducing Cr（Ⅵ）to immobile Cr（III）,which could affect the concentration and speciation distributions of chromium in soil washing eluents and subsequently the treatment efficiency of chromium removal.Meanwhile,due to the adsorption properties of LDH materials to Cr（Ⅵ）,they have been regarded as a group of new functional materials that can be applied in wastewater treatment processes such as sand filtration.Therefore,this thesis focuses on the chromium-containing wastewater generated during the soil washing remediation of chromium contaminated sites,taking the common soil minerals:Zn/Al LDH and Fe/Al LDH nano-minerals in soil washing eluents as the main research objects.Batch experiments and column studies using quartz sand as the packing medium with different characterization methods were used to reveal the effect of these LDH nano-minerals on the adsorption mechanism of Cr（Ⅵ）and removal of Cr（Ⅵ）in sand filtration system.The effects of different hydrodynamic and environmental factors on the migration of Cr（Ⅵ）were investigated.On the basis of the above research,the heterogeneous sand filtration structure of the actual site was simulated to study the effect of LDH on the actual sand filtration system.The investigation on the effects of different physicochemical and morphological properties of LDHs on the transformation and removal of Cr（Ⅵ）can be applied to the evaluation of the removal of chromium-containing wastewater generated during wet remediation of chromium contaminated soils.The specific research results are as follows:（1）The adsorption of Cr（Ⅵ）on the nano-Zn/Al LDH is controlled by p H.With the increase of p H,the positive charge on the surface of Zn/Al LDH gradually decreases,and the adsorption of Cr（Ⅵ）decreases.At p H>p H_pzc,there is still a part of Cr（Ⅵ）adsorbed on the surface of Zn/Al LDH,indicating that the adsorption of Cr（Ⅵ）is not only electrostatic adsorption,but also chemical adsorption.The results of Cr（Ⅵ）and Zn/Al LDH co-elution column experiments showed that in addition to p H,the electrolyte compositions including both cation and anion types,as well as their concentrations had significant effects on the migration of Cr（Ⅵ）.The anion exchange on LDH and LDH colloid-facilitated transport accounted for the migration of Cr（Ⅵ）in the quartz sand column.The Colloid-facilitated contaminant transport model can well describe the breakthrough curves and spatial retention curves of Zn/Al LDH and Cr（Ⅵ）in the column study.（2）The batch experiments results show that Fe/Al LDH has a stronger fixation effect on Cr（Ⅵ）than Zn/Al LDH,and the fixation mechanism is combined reduction-adsorption mechanism.In the range of p H=5～11,the removal rate of Fe/Al LDH to Cr（Ⅵ）did not change much,and the removal rate reached 99.51%and 77.87%for 10 mg/L and 20 mg/L Cr（Ⅵ）at p H=5,respectively.With the increase of p H,The adsorption capacity of Cr（Ⅵ）on Fe/Al LDH gradually decreased.After Cr（Ⅵ）adsorbed on Fe/Al LDH,both valence states of Fe and Cr changed,specifically,Fe（II）content decreased from 63.75%to 22.15%,and Fe（III）content increased from 36.25%to 77.85%while 27.05%of Cr（III）was produced,indicating that the redox reaction between Cr（Ⅵ）and Fe（II）occurred on the surface of the LDH mineral.Solution chemistry has a significant effect on Fe/Al LDH-controlled Cr（Ⅵ）migration in Fe/Al LDH-coated quartz sand column study.The two-site colloid-faciliated contaminant transport model can well describe the breakthrough curves and spatial retention curves of Fe/Al LDH and Cr（Ⅵ）.（3）The presence of colloids and anions such as SO₄^2-ions in the soil eluent significantly affected the removal of Cr（Ⅵ）by sand filtration,which made Cr（Ⅵ）more easily migrated.Zn/Al LDH and Fe/Al LDH coated quartz sand inhibited the migration of Cr in sand filtration.Under the same conditions,Fe/Al LDH-coated quartz sand has higher removal efficiency for Cr（Ⅵ）,and the maximum solid phase retention concentration S_max2 is 2.97-88.9 times higher than that of Zn/Al LDH,and as the LDH coating amount increased from 30 mg to 300 mg,significantly improved Cr（Ⅵ）removal efficiency was obtained.The study proves that under the unfavorable conditions in presence of large amounts of colloids and anion components in the soil eluent,the addition of Fe/Al LDH coating to the sand filtration system can still effectively remove Cr（Ⅵ）through adsorption and reduction immobilization mechanisms,which provides a theoretical basis for the improvement of wastewater treatment for soil washing eluent solutions.