Characterisation of Mg-Al Hydrotalcite and Surfactant-Modified Bentonite Nano Clays for the Treatment of Acid Mine Drainage

Abstract

The presence of acid mine drainage (AMD) poses health risks to humans and other living organisms. While much research on AMD has been conducted, the absence of a widely accepted AMD treatment technology makes it an ongoing topic for further exploration. The present study explored the potential of nano-clay adsorbents for the removal of iron and manganese from AMD. The two nano-clay adsorbents used in this study were synthetic hydrotalcite (HT) and modified bentonite (MB) clays. The adsorption media were characterised before and after adsorption using X-ray diffraction (XRD), Fourier transforms infrared (FTIR) spectroscopy, Brunauer–Emmett–Teller (BET), and Scanning Electron Microscope (SEM) to explain the adsorption mechanisms. To investigate the efficiency of the clays, batch adsorption studies were carried out by varying the concentration, pH, and time. To establish the kind of adsorption mechanism that occurred, Langmuir and Freundlich isotherms were applied. It was found from characterisation results that HT and MB contained hydroxyl and carbonyl functional groups responsible for surface complexation mechanisms; XRD showed that isomorphic substitution and precipitation also occurred in adsorption using HT. The specific surface area for modified bentonite and hydrotalcite as determined by BET was 3.13 and 44.7 m2/g respectively. The XRD and the SEM results showed that HT was crystalline while MB was amorphous, probably due to the irregular stacking of the surfactant. It was observed that the adsorbent deprotonated as pH increased, resulting in an increase in metal removal. The Langmuir isotherm provided the best monolayer adsorption capacity with RL < 1 and correlation coefficients ranged from 0.95 to 0.99 for both adsorbents.