Clinoptilolite modified by calcium and hydroxyl ions for removal of fluoride from aqueous solution

Abstract

In Mexico naturally occurring fluoride contamination of groundwater is widespread. Adsorption with local and low-cost materials has been recently studied. In this research, clinoptilolite was modified with hydroxyl, calcium, and hydroxyl followed by calcium to compare their fluoride removal capacity. The sorption of natural and modified materials was investigated for the initial fluoride concentrations from 1.5 mg/L to 15 mg/L, isotherms for these adsorbents were investigated and techniques such as scanning electron microscopy (SEM) and X-ray diffraction (XDR) were used to characterize these adsorbents. It was found that removal fluoride was highest with calcium modified clinoptilolite from 60.2% with 1.5 mg F-/L to 43.0% with 15 mg F-/L. Sorption data for calcium modified clinoptilolite and hydroxyl and calcium modified clinoptilolite was found to fit Freundlich equations with equilibrium constants of 0.12 L/g and 0.021 L/g, respectively. SEM and XRD showed no significant changes of clinoptilolites after and before modification with calcium and sorption of fluoride, suggesting that interaction between adsorbent and adsorbate were only superficial. Modification of clinoptilolite with calcium enhanced its capacity for fluoride removal. This modification process was carried out using an economical and abundant mineral in central Mexico, which gives an additional advantage to the process.