Sulfate removal by Mg–Al layered double hydroxide precipitates: Mechanism, settleability, techno-economic analysis and recycling as demulsifier

This study investigated sulfate (SO42−) removal by Mg–Al hydrotalcite-type (HT) compound (Mg6Al2SO4(OH)16•mH2O), and analyzed applicable condition of the method from techno-economic perspective. Under optimized condition of pH 8.80, molar ratio of Mg2+ and SO42− (Mg/S ratio) of 6.63 and molar ratio of Al3+ and SO42− (Al/S ratio) of 2.53, HT compound achieved a greatly lower concentration of SO42− (3.76 mmol/L) than conventional gypsum precipitation. The composition and morphological analysis of precipitates showed that HT compound was transformed to Mg(OH)2 at pH > 9.0, and Mg(OH)2, Al(OH)3 and MgAl2(OH)8 at Al/S ratio > 2.5. HT compound had better settleability than Mg(OH)2 and ettringite, and was easier to be flocculated by anionic polyacrylamide to obtain a lower SV30 of 16.4%. Turbiscan showed that HT compound had the highest instability and aggregation property among the three systems. Techno-economic analysis showed that HT compound coupled ettringite route was more economic than Mg(OH)2 coupled ettringite route at Mg/S ratio > 6.08, but was not applicable when concentrations of Mg2+ and SO42− were very low. The precipitated HT compound was recovered as demulsifier and showed more efficient COD removal than conventional demulsifier for chemical cleaning wastewater treatment. [Display omitted] •SO42− removal by hydrotalcite-type (HT) compound (Mg6Al2SO4(OH)16.•mH2O) was studied.•SO42− was reduced to 3.76 mmol/L at pH 8.80, Mg/S ratio 6.63 and Al/S ratio of 2.53.•HT compound had better settleability and aggregation than Mg(OH)2 and ettringite.•HT-Ett route was more economic than Mg(OH)2-Ett route at Mg/S ratio > 6.08.•HT compound was recovered as demulsifier with efficient COD removal from wastewater.