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

Abstract

Sulfate (SO₄^2-) 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 SO₄^2- from synthetic and real groundwater having 3901 mg(SO₄^2-)/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 SO₄^2- 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 SO₄^2- 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 SO₄^2- 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 SO₄^2- from real groundwater and can be utilized as a pretreatment unit for high sulfate water resources.

Keywords: Activated carbon; Adsorption kinetics; Characterization; Groundwater; Layered double oxide; Sulfate removal.