Removal of fluoride and arsenate from aqueous solution by hydrocalumite via precipitation and anion exchange

• A wide range of solution F or As concentrations could be reduced by hydrocalumite to below DWS.
• The primary mechanism for F or As removal is formation of F- or As-bearing minerals.
• The efficiency of water defluoridation by hydrocalumite is better than HTlcs or calcium salts.
• Hydrocalumite is promising for treatment of high fluoride or arsenate water.

Hydrocalumite was used to treat fluoride and arsenate solutions with a wide range of initial concentrations. A marked decrease in solution fluoride and arsenate to below 0.05 and 0.005 mmol/L was observed when their initial concentrations were not higher than 30 and 5 mmol/L respectively. However, a sharp increase occurred to equilibrium F and As concentrations over these critical initial concentrations. The mechanisms for fluoride and arsenate removal were investigated by comparison between measured chemical compositions of solution samples and their PHREEQC simulation along with XRD, SEM and EDX analyses for solid samples. The solution defluoridation was primarily a result of hydrocalumite dissolution and subsequent precipitation of fluorite and fluoride-bearing bayerite, although the anion exchange between fluoride in solution and chloride in interlayers of hydrocalumite functioned as well provided that not all hydrocalumite added to solution was dissolved. In contrast, the principal approach for arsenate removal was through the formation of johnbaumite and other arsenic-bearing minerals or solid–solution series. Solution pH value has little effect on defluoridation and dearsenication by hydrocalumite due to its pH buffering effect, while the coexistence of competitive anions and the variation of reaction temperature affected the fluoride and arsenate removal significantly. The maximum uptake capacities of hydrocalumite for fluoride and arsenate are 719.1 and 361.7 mg/g respectively, much better than other types of layered double hydroxides. It is promising for treatment of not only naturally occurring poor-quality groundwater with not very high fluoride and arsenic concentrations but also industrial wastewater enriched highly in fluoride or arsenic.