Optimizing a low added value bentonite as adsorbent material to remove pesticides from water

A local low value bentonite from Southern Spain (Raw Bentonite), previously decarbonated (Bent), was modified to improve its pesticide adsorption capacity with Fe³⁺, hexadecyltrimethylammonium (HDTMA) and the biopolymer chitosan (CH). Adsorption of pesticides on powdered samples showed that Fe³⁺ and HDTMA were appropriate modifiers for this purpose. The modification was optimized by saturation with Fe³⁺ (Bent-Fe) and hexadecyltrimethylammonium (Bent-HDTMA) and the obtained adsorbents were characterized by several physicochemical techniques (X-ray diffraction, thermogravimetric analysis, X-ray fluorescence, physisorption of N₂). Their adsorption capacity to remove three widely used pesticides in Andalusian crops (terbuthylazine, tebuconazole and MCPA) from water was assessed and compared with the commercial organoclay Cloisite® 10A (Clo10). The modified bentonites adsorbed the selected pesticides in a percentage ranging from 30 to 100%, whereas sorption on Clo10 ranged from 30 to 90%. For their possible use as filtering beds, Bent-HDTMA, Bent-Fe and Clo10 were granulated by using three different binders (colophony resin and carnauba and bee waxes) at three different mixing ratios and the water resistance and pesticide adsorption of the granules were measured. Results showed that the granulation process did not alter the pesticide adsorption capacity of the powdered modified bentonites, and both waxes granules (carnauba and bee wax) showed better behavior than resin granules. In this work, we succeeded in the preparation of granulated adsorbents derived from a low cost material with similar behavior against a high purity smectite (precursor of Cloisite® 10A). This raises an alternative for this waste material to be used in filter systems for removing pesticides from contaminated water.