Removal of pesticides from aqueous solutions by combined physicochemical processes—the behaviour of lindane

The adsorption of the insecticide lindane on powdered activated carbon (PAC) from aqueous solutions with and without the simultaneous presence of coagulants has been studied. Ferric chloride and basic polyaluminium chlorosulfate were examined as coagulants, in laboratory experiments aided in certain cases by polyacrylamide (a common polyelectrolyte) addition, in order to evaluate and optimize the various process parameters, such as coagulant and carbon concentrations, solution pH and position of coagulant addition (before or after PAC). The obtained results indicated that PAC adsorption efficiency was not highly affected by solution pH, while PAC doses greater than 20 mg/L were found to be necessary in order to reduce lindane from initial concentration of 10 μg/L down to 0.1 μg/L within 1 h contact time. However, when coagulants have been added after the addition of PAC, a certain decrease of adsorbent efficiency was observed and, therefore, double PAC concentration (40 mg/L) was necessary for the aforementioned removal. In order to evaluate the effect of position of coagulant addition on the removal efficiency of PAC, the respective adsorption isotherms were calculated and rate studies were performed.