Adsorption Of Metals By Nanoscale Amendments In Solution And Its Application In Remediation Of Cd-polluted Soil

Preparation of high efficient and environment-friendly amendments and their potential applicationin remediation of polluted soils and waters have received wide attention in the last decade. With the fastdevelopment of environmental molecular material science and engineering, the application of nanoscalematerials in remediation of polluted soils and waters has gained more and more attention recently. Withthe characteristics of enhanced huge planar surface, nanoscale amendments are expected to enable theincreased interface reactions, e.g. the surface and specific adsorption of metal ions and organicpollutants onto the sorbent, the oxidation-reduction reaction on the planar surface etc. It can bepredicted that the nanoscale amendments will play more and more important roles in the remediation ofpolluted soils and waters, but caution is required as to application of nanoscale amendments (<100nm)in environment. Studies relevant to organism exposure have shown that some nanoscale amendmentscan be hazardous. In this dissertation, the preparation and characteristion of different nanoscaleamendments were studied, adsorption and removal mechanism of aqueous metal ions by nanoscaleamendments in solution and soils were investigateded respectively, the short-term effects of nanoscaleamendments on Cd phyto-toxicity to root growth of four plant species were investigated as based on theISO toxicity test protocol, both the nanoscale amendments and Cd influence on growth of the plant weretested, with the aim to promote the remediation effiency of polluted soil with high efficient andenvironment-friendly nanoscale amendments. The main conclusions mainly include:1. The magnetic Fe₃O₄nanoscale amendment was prepared using modified chemicalco-precipitation methods, and corresponding thiolated and coating nanoscale amendments (Fe₃O₄,Fe₂O₃(α and γ))were also obtained via surface mercaptopropyltriethoxysilane (MPTES) modificationand humic acid (HA). The properties and structure of the nanoscale amendments were characterized byX-Ray diffraction (XRD) and infrared spectroscopy (IR) etc. The results showed that the average size ofprepared nano-Fe₃O₄, Fe₂O₃(α and γ)were about20-40nm with homogeneous distribution, differentfuctionalized treatments can increase the average size of prepared nanoscale amendments; IR showedthe-SH and C=O stretched were found in the surface of functionalized nanoscale amendments; XRDindicated that there was no changes between naked and functionalized nanoscale amendments in thecharacteristic absorption peaks, and these peaks are suitable with stereo crystal planes of nanoscaleamendments;2. The application of the thiolated and HA coating nanoscale amendments prepared above foradsorption and removal of aqueous Pb²⁺, Cd²⁺, Cu²⁺in stable temperatures (25±2℃) were alsoinvestigated in this study. The results showed these nanoscale amendments can absorb Pb²⁺, Cd²⁺, Cu²⁺in solution efficiently and the adsorption data were well fitted with the Langmuir isotherm. Theadsorption of naked FexOyand FexOy/MPTES to metal ions followed the order Pb²⁺>Cd²⁺>Cu²⁺, butFexOy/HA followed the order Pb²⁺>Cu²⁺>Cd²⁺; the coating of humic acid with FexOynanoscaleamendments can obviously improve its adsorption capacity for Cd²⁺and Cu²⁺in solution, but marginal effect was observed for Pb²⁺;3. A pot experiment was conducted to study the effects of different nanoscale amendments(hydroxyapatite, red mud, Fe₃O₄, humic acid (HA)-Fe₃O₄) on the uptake and transfer factors of Cd bycarrot in two kinds of Cd-polluted soils. The results showed that nanoscale amendments mentionedabove could increase plant biomass and tolerance index (TI) of carrot significantly. The application ofnanoscale amendments could decrease the concentration of Cd of different carrot parts, the Cdconcentrations in both shoots and roots of carrot decreased with increasing quantities of addednanoscale amendments. The Cd concentrations in shoots of carrot decreased78.8%, and in roots67.8with the most scope respectively, as compared with the control treatment; The application of nanoscaleamendments in polluted soils could decrease transfer factors (TF) and biological concentration factors(BCF) of Cd by carrot in different extent, the mechanism may probably due to the transformation of soilCd from nonresidual fractions to residual fraction substantially after addition of the nanoscaleamendments. In general, the efficiency of different amendments on the Cd bioavailability reductionfollowed this order at the equivalent additions: RM^HAP>HA-Fe₃O₄> Fe₃O₄;4. The short-term effects of six types of nanoscale amendments (Kaolin, montmorillonite,hydroxyapatite, Fe₃O₄, α-Fe₂O₃, γ-Fe₂O₃) on the EC50s (Cd) for root growth of four plant species (i.e.tomato, cucumber, carrot and lettuce) were investigated using standard toxicity testing. nanoscaleamendments and Cd influencing on growth of the plant were as well as tested, respectively.Scanning-electron microscopy (SEM) equipped with the element dispersive spectrometer (EDS) wasused to observe the interaction of nanoscale amendments prepared with EC50s (Cd) as the solvents withthe root surface and identify the mechanisms of Cd toxicity reduction to the root growth induced bynano-amendments additives. The results showed that the seedling growth was negatively related to theexposure concentration of Cd, among the tested plants, the sensitive endpoint appeared in the order oftomato> carrot≈lettuce> cucumber according to the ECx measured. The root growth was notsignificantly inhibited by the presence of nanoscale amendments except for HAP on tomato, but wasnoticeably promoted by particular nanoscale amendments suspensions prepared with EC50s (Cd) as thesolvents at higher test concentrations compared with the controls (Cd, EC50s) with one exception forKaolin. Microscopy images showed roots of tested plants exposed to Cd exhibited a decrease in rootdiameter and root wilt, and the disintegration of the root epidermis, the clutter root surface showed theevident stress under Cd solution, after the addition of nanoscale amendments, many root hairs and nodisintegration on the surfaces of the root system can be observed, nanoscale amendments crystal alsooccurred on the plants root surface. The element dispersive spectrometer (EDS) analysis showed that theprecipitation mainly contributed to phytotoxicity reduction by the nanoscale amendments.