Removal of toxic anions and organic pollutants by solid phase extraction using low cost biological, geological and polymeric adsorbents

Abstract

Water pollution is one of the major problems in under developed countries. Due to limited resources, clean water is not available to a significant part of population in Pakistan. Industrial effluents, municipal waste, use of pesticides and insecticides are the principal sources of contamination even for underground water. Heavy metals, organic pollutants, toxic anions and biologically hazardous compounds are constantly polluting our fresh water bodies. In last few years, much attention has been paid to remove heavy metals from waste water by different group of researchers working in universities and research and development organizations of the country. As adsorption is the simplest cheap and fast technique as compared to other sophisticated technologies, various adsorbents were employed to decontaminate water sources in order to utilize water for various purposes. In present studies, adsorptive removal of toxic anions (sulfide, fluoride, cyanide, arsenate and chromate) and organic pollutants (phenol and Alizarin S) has been studied as these pollutants are commonly added into water bodies through industrial effluents in Pakistan. Various indigenous and easily available adsorbents from biological, geological and polymeric origin were selected for this purpose. Biological adsorbents included rice husk, wheat straw, along with some indigenous plant materials like Kikar leaves, Bhindi stem, Arjun nuts and Beerri ptta capsule. Bentonite, kaolin, and indigenously available cheap clays: multani mitti and gachni were used among geological adsorbents. From third category cotton was used as natural whereas cellulose, bakelite and amberlite IRA 410 were employed as synthetic polymeric adsorbents. Before conducting batch wise adsorption studies, various concerned physiochemical parameters like pH, porosity, and moisture/ash content and iodine number of each adsorbent were recorded. Surface morphology, quantitative composition information of the elements and functional groups studies for both raw and used adsorbents was conducted by; Scanning Electron Microscopy (S.E.M) and Energy Dispersive X-Ray spectroscopy (EDX), Fourier Transform Infrared Spectroscopy (FT-IR). Structure of the clays was further studied by XRay Diffraction (XRD) technique. Elemental composition of the adsorbents was also verified using CHNS elemental analyzer. After conducting the surface study, initial test experiments were performed using each adsorbent for selected adsorbates. Those adsorbents giving less than 25% removal efficiency were excluded from the adsorption study of that adsorbate. Hence detailed adsorption studies for each pollutant were made with selected adsorbents. In order to get better results a few adsorbents were also chemically modified by using various chemical reagents. During the adsorption studies; parameter like; time of contact, agitation speed, adsorbent dose, pH and temperature were optimized. Isothermal, Kinetics and Thermodynamical studies were carried out using the optimized conditions. After adsorption studies desorption was also investigated by using various chemical reagents. Sulfide was effectively removed by Kikar leaves Bentonite and Amberlite IRA 41, whereas Wheat straw, Arjun nuts, Gachni clay, Cotton and Amberlite IRA 410 showed good response for removal of fluoride. Cyanide was better eliminated by Arjun nuts, gachni clay, cellulose and amberlite IRA 410. Chemically modified adsorbents were found good for the adsorption of arsenate and chromate. Kikar leaves, arjun nuts, gachni and cotton efficiently removed phenol from water. Arjun nuts, Kikar leaves, Multani mitti and Amberlite IRA 410 effectively eliminated alizarin Red S dye. Absorption of all these pollutants followed Langmuir monolayer adsorption pattern with the rate dependence on pseudo second order kinetics model. Thermodynamical study revealed that that adsorption in all cases was spontaneous and exothermic in nature except arsenate and chromate in which adsorption was found endothermic. FT-IR spectra, S.E.M and X-ray studies supported the physiochemical adsorption between the adsorbents and different pollutants. Desorption studies showed that adsorbents can be regenerated by using appropriate chemical reagents. Present research work showed that anions and organic pollutants can be effectively eliminated from polluted waters by employing indigenously available cost effective adsorbents.