Effect of pore clogging on kinetics of lead uptake by clinoptilolite

The kinetics of lead-sodium ion exchange using pretreated natural clinoptil olite are investigated, more specifically the influence of agitation (0, 21 0, and 650 rpm) on the limiting step of the overall process, for particle s izes of 0.63-0.8 and 0.8-1 mm at ambient temperature and initial lead solut ions of 500 mg l(-1) without pH adjustment. The isotopic exchange model is found to fit the ion exchange process. Particle diffusion is shown to be th e controlling step for both particle sizes under agitation, while in the ab sence of agitation film diffusion is shown to control. The ion exchange pro cess effective diffusion coefficients are calculated and found to depend st rongly on particle size in the case of agitation at 210 rpm and only slight ly on particle size at 650 rpm. Lead uptake rates are higher for smaller pa rticles only at rigorous agitation, while at mild agitation the results are reversed. These facts are due to partial clogging of the pores of the mine ral during the grinding process. This is verified through comparison of lea d uptake rates for two samples of the same particle size, one of which is r igorously washed for a certain time before being exposed to the ion exchang e. (C) 1999 Academic Press.