The use of compost - Red Gypsum mixture as a low cost alternative adsorbent for lead

Red gypsum, a waste material which derives from the production of titanium dioxide with a high content of CaSO4 and Fe(OH)2, was evaluated as a low-cost adsorbent mixed with green compost. Red gypsum is becoming interesting because of the widespread TiO2 industrial production in many industrialized countries. The aim of this work was to evaluate whether a mixture of red gypsum and compost could be used as an adsorbing material to decontaminate waters polluted with heavy metals. Batch adsorption experiments were carried out with Lead as representative of heavy metals in polluted waters. Lead adsorption was studied to test the performance of the composite material as a function of the composition of the mixture. The evaluation of the adsorption capacity of the compost plus red gypsum mixture was based on the Freundlich and Langmuir equations. The results indicate that the composite material has a high adsorption capacity for Pb from aqueous solutions. The adsorption capacity grew following the addition of increasing amounts of red gypsum due to the increase in negative charges, which promoted the attraction towards the positively charged Pb ions. Pb adsorption may occur as a result of different mechanisms such as ion exchange, surface complexation and electrostatic interaction. The mixture compost - red gypsum showed a considerable capacity to remove Pb from aqueous solutions over a wide range of concentrations.