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A study of selenate removal from water by adsorption on an aged, carbonated Friedel’s salt material with mass loss considerations Rasouli, Maryam

Abstract

In this work, selenate removal from water was studied using a material from the layered double hydroxide family. This material is a mixture of two hydrocalumite phases, chloro-hydrocalumite [Ca₄Al₂(OH)₁₂]·[Cl₂·4H₂O] and chloro-carboaluminate hydrocalumite [Ca₄Al₂(OH)₁₂]·[Cl(CO₃)₀.₅·10.8H₂O]. This material is called aged-FS in this thesis. Aged-FS was obtained by the co-precipitation method followed by an aging process. The aging condition was initial pH 12.0, liquid/solid ratio 20, 65 °C, for 7 days. During aging, a phase transformation from chloro-hydrocalumite in the hexagonal crystal system to chloro-carboaluminate hydrocalumite in the monoclinic crystal system was observed. The aluminum hydroxide impurity was removed and katoite (Ca3Al2(OH)12) was formed. The formation of well-shaped hexagon crystallites and agglomeration of particles were observed by morphology and particle size studies, respectively. The carbonate content increased during aging from 1.15 to 3.00 wt.%. By aging, ~50% reduction in FS dissolution was recorded. By some modifications in nitrogen purging of the synthesis process, the carbonate content was reduced from 1.15 to 1.01 wt.%. For selenate removal studies, the effect of initial pH, liquid/solid ratio, time, temperature, agitation, selenate concentration, sulfate and carbonate interference, and container type were studied. The highest loading percentage of selenate and the lowest dissolution of aged-FS was achieved at initial pH 12.0, liquid/solid ratio 200, 30 °C, 24 h, and 150 RPM. A selenate loading isotherm was fitted to the Langmuir model with maximum loading of 71.1 mg Se/g. The intercalated carbonate in aged-FS did not interfere with selenate removal while significant interference was observed from carbonate in solution. The sulfate anion did not interfere with selenate adsorption. The Al dissolution from aged-FS in an adsorption test was suppressed using calcium hydroxide solution. The Al concentration reached to 0.07 ppm; a value lower than the maximum instantaneous value set by a BC guideline. Calcination of aged-FS improved the selenate loading. The lowest level of selenium in solution achieved using the calcined aged-FS (1.84 ppb) met the 2016 US EPA maximum discharge limit of lotic water for Se (

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