Chen, Dan Ting; (2024) Speciation of Toxic Pollutants in Hazardous Industrial Wastes by X-ray Absorption Spectroscopy. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Metal finishing filter cake, Pb-Zn smelter slag and municipal waste combustion air pollution control residue (APCR) are hazardous industrial wastes from metal-finishing, smelting and municipal solid waste incineration. The speciation of toxic elements in these wastes controls their recovery or fate in the environment but has been little investigated. The doctoral research developed a step-by-step approach of using data from As, Cu, Cr, Ni and Zn K edge and Pb L3 edge X-ray Absorption Spectroscopy (XAS), supported by Ca, Fe, P, S and Si K-edge XAS and other techniques, to identify and compare element speciation in samples of the three types of waste. In the two investigated filter cakes (TX and ST), Cr and Fe co-precipitated as (CrxFe1-x)2O3∙yH2O. Ni was present as Ni(OH)2 and NiCO3·2Ni(OH)2·4H2O. Zn was present as poorly crystalline 2ZnCO3∙3Zn(OH)2 and Zn3(PO4)2. Pb was adsorbed on amorphous SiO2. Cu precipitated as Cu2Cl(OH)3, Cu(OH)2, and CuSO4·5H2O in ST, whereas Cu appeared to adsorb on amorphous SiO2 in TX which contained much less Pb. In Pb/Zn slag, arsenic speciation was likely dominated by arsenic (III) and (V) oxides and sulfide. Cu was mainly speciated as Cu2S, with some metallic Cu and a weathering product, Cu(OH)2. Pb was identified mainly in glass and apatite (e.g., Pb5(PO4)3OH). Zn was mainly present in glass, ZnS and possibly solid solutions with Fe oxides. In raw and water washed APCR, Cu was found mainly in Cu2Cl(OH)3 and CuO, followed by Cu in glass. Pb was identified mainly in glass, with some PbSO4 and a small portion of PbO. Zn was mainly present in Zn4Si2O7(OH)2∙H2O, 2ZnCO3∙3Zn(OH)2, and solid solutions with spinel Fe3O4. The application of XAS in the three wastes, following the step-by-step approach, significantly improved the understanding of speciation of the elements of interest at relatively low concentrations in a heterogeneous matrix, regardless of the crystallinity.

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