Sorption of Chromate and Arsenate by Surfactant Modified Zeolite

In recent years growth of industries has led to introduction of pollutants in nature, especially the heavy metal ions like Cr(III) and As(V) usually present in many waste water. Heavy metal containing waste is generated from industries like metallurgical, mining, chemical, leather, distilleries, sugar, battery, electroplating and pigments. Chromium is found in natural water in oxidation states Cr(III) and Cr(VI). The former is essential elements for mammals where as the later is reported to be toxic. Same as arsenic is an ubiquitous contaminant which can be found at high concentrations in natural waters and wastewaters. Arsenic containing waste streams are generated mainly by the microelectronic industry and by pesticide and pharmaceutical facilities. It occurs in two oxidation states that form oxyanions, arsenate As(V), and arsenite As(III); arsenite is more mobile and toxic than AsO₄ ^-3. As arsenic is a major hazardous contaminant for the human health, its removal is an issue of increasing concern. Zeolite are crystalline, hydrated aluminosilicate containing exchangeable alkaline and alkaline earth cations in their structural frameworks. Since zeolite have permanent negative charge on their surfaces, they have no affinity for anions. However recent studies have shown that modification of zeolite with certain surfactants or metal cations yield sorbents with a strong affinity for many anions. Zeolite has high internal and external surface areas and high internal and external cation exchange capacities suitable for the surface modification by cationic surfactant (HDTMA). When the initial surfactant concentration is less than the critical micellar concentration (CMC), the sorted surfactant molecules primarily form a monolayer, limited chromate and arsenate sorption indicates that the patchy bilayer may also be formed. When the surfactant concentration is greater than the critical micellar concentration and enough surfactant exist in the system, the sorbed surfactant molecules form bilayer, producing maximum chromate and arsenate sorption. Quantitative sorption of chromate and arsenate and desorption of bromide ions indicate that the sorption of oxoanions is primarily due to surface anion exchange.