Removal of cadmium from aqueous solutions by palygorskite

The sorption characteristics of palygorskite with respect to cadmium were studied with the aim of assessing its use in water purification systems. Using a batch method the influence of time (0.5-48 h), initial Cd concentration (5-150 mg/l or 0.044-1.34 mmol/l), ionic strength ([Ca(II)]: 0-0.1 mol/l), pH (3-7) and mineral dose (1-20 g/l) on Cd removal was evaluated. The sorption of Cd on palygorskite appeared as a fast process, with equilibrium being attained within the first half an hour of interaction. This process could be described by the Langmuir model and gave a maximum Cd sorption of 4.54 mg/g. This sorption capacity value was greatly affected by both pH and ionic strength. Thus, Cd sorption decreased as initial pH lessened, especially at proton concentrations similar to those of Cd, at which competition for variable charge sites (silanol groups on palygorskite surface) appeared to be important. High competing electrolyte concentrations also decreased significantly (close to 60%) the amount of sorbed Cd, suggesting a great contribution of replacement of exchange cations in this metal removal by palygorskite. The increase of mineral dose provoked a Cd removal raise; removal values in the range 85-45% were attained for Cd initial concentrations of 10-150 mg/l (0.089-1.34 mmol/l) when a palygorskite dose of 20 g/l was employed. Column studies were also performed in order to estimate the potential of palygorskite to be used in continuous flow purification systems, showing the effectiveness of this mineral to purify down to the legal limit of waste moderate volumes of Cd-containing solutions with a similar concentration (50 mg/l or 0.445 mmol/l) to those mostly found in the upper range of concentrations usually present in industrial wastewaters.