Comparison of adsorbent materials for herbicide diuron removal from water

Diuron herbicide is a persistent and frequently detected compound in surface and groundwater. In this work, a comparative study of the performance of four types of adsorbent (two precipitated silica, zeolite Y and carbon molecular sieves (CMS)) for removing diuron from water was carried out. Precipitated silica samples were obtained by controlled precipitation of SiO2 with sulphuric acid from water glass; zeolite Y was synthesized by a microwave-assisted hydrothermal method; CMS were synthesized by the method of controlled pyrolysis of wheat straw and chemical vapour deposition of organic matter. It was found that adsorption isotherms for precipitated silica and zeolite type Y are of linear shape, whereas the one for CMS exhibits at low concentrations a concave shape followed by an inflection point which suggests cooperative adsorption and the formation of adsorbate multilayers. CMS adsorbent showed the highest adsorption capacity and this was likely due to its high hydrophobicity. Temperature has a negligible effect on precipitated silica and zeolite Y while it strongly affects adsorption properties of CMS. Adsorption is not thermodynamically favoured at high temperature. An unexpected decrease in the rate of adsorption was observed with increase in temperature. This is possibly related to the different adsorption behaviour of diuron conformers and aggregates.