Exploration of As(III)/As(V) Uptake from Aqueous Solution by Synthesized Calcium Sulfate Whisker

Although common calcium-containing minerals such as calcite and gypsum may fix arsenic, the interaction between modified calcic minerals and arsenic has seldom been reported. The uptake behavior of As(III)/As(V) from aqueous solutions by calcium sulfate whisker (CSW, dihydrate or anhydrite) synthesized through a cooling recrystallization method was explored. A series of batch experiments were conducted to examine the effect of pH, reaction time, whisker dosage, and initial As concentration. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the samples prepared. The results showed that pH of the aqueous solution was an important parameter for As(III)/As(V) uptake, and an excellent removal efficiency could be achieved under strongly alkaline condition. The data from batch experiments for reaction of As(V) with calcium sulfate dihydrate whisker (CSDW) and calcium sulfate anhydrous whisker (CSAW) were well described with extended Langmuir EXT1 model, from which theoretic maximum adsorption capacity of 46.57 mg As(V)·(g CSDW)− 1 and 39.18 mg As(V)·(g CSAW)− 1 were obtained. Some calcium arsenate solids products, such as CaAsO3(OH) (weilite, syn), Ca3(AsO4)2 (calcium arsenate), CaO–As2O5, Ca–As–O, Ca5(AsO4)3OH·xH2O (calcium arsenate hydroxide hydrate), and CaH(AsO4)·2H2O (hydrogen calcium arsenic oxide hydrate), were detected at pH = 12.5 through XRD analysis. This indicates that the interaction mechanism between As(V) and CSW is a complex adsorption process combined with surface dissolution and chemical precipitation.

The reaction of arsenic and calcium sulfate whiskers (CSWs) synthesized through a cooling recrystallization method was explored. The experimental data for a wide range of As(V) concentrations (from 1 to 300 mg·L− 1) at pH of 11 are well fitted with the extended Langmuir EXT1 model. CSW shows its potential for arsenic removal. SEM images of reacted CSW exhibited partial dissolution and some Ca–As compounds were detected from XRD spectrum. These facts indicate that the interaction process of arsenic and CSW is not pure adsorption but a combination of adsorption and surface dissolution–precipitation.Figure optionsDownload as PowerPoint slide