Novel composites of activated carbon and layered double oxide for the removal of sulfate from synthetic and brackish groundwater

Sulfate (SO₄²⁻) is a major water and environmental concern that causes severe diarrhea, death of invertebrates and plant species, and clogging of industrial pipes. In the current work, treatment of SO₄²⁻ from synthetic and real groundwater having 3901 mg(SO₄²⁻)/L was investigated for the first time using Zn–Al and Mg–Al layered double oxides doped granular activated carbon (GAC/Mg–Al LDO and GAC/Zn–Al LDO). The co-precipitation method was followed to synthesize the GAC/LDO composites using an Mg or Zn to Al molar ratio of 3:1. The GAC/Mg–Al LDO possessed a higher specific surface area (323.9 m²/g) compared to GAC/Zn–Al LDO (195.1 m²/g). The GAC/Mg–Al LDO demonstrated more than 99% removal of SO₄²⁻ from synthetic water, while it was 50.9% for GAC/Zn–Al LDO and less than 1% for raw GAC at an initial concentration of 50 mg/L. The GAC/Mg–Al LDO was selected for further batch experiments and modeling investigation. The equilibrium data followed the Redlich-Peterson and Langmuir models with determination coefficients of 0.943 and 0.935, respectively. The maximum Langmuir adsorption capacity was 143.5 mg/g. In the real groundwater adsorption study, the screening experiment revealed high selectivity towards SO₄²⁻ with 62% removal efficiency. The optimum dosage was found to be 50 g/L with an uptake capacity of 61.5 mg/g. The kinetic data of SO₄²⁻ removal from synthetic and brackish water were in excellent agreement with the pseudo-second order model, and the equilibrium was attained in 5 h. Accordingly, it can be concluded that the GAC/Mg–Al LDO is an efficient material for treating SO₄²⁻ from real groundwater and can be utilized as a pretreatment unit for high sulfate water resources.