279 J. Indian Chem. Soc., Vol. 95, March 2018, pp. 279-284 Removal of selenium from drinking water by adsorption Kamakshi Singh, Poonam Tirkey and Tanushree Bhattacharya* Birla Institute of Technology, Mesra-835 215, Ranchi, Jharkhand, India E-mail : kamakshi.1991@gmail.com, poonamtirkey04@gmail.com, tbhattacharya@bitmesra.ac.in Manuscript received 15 November 2017, revised 06 March 2018, accepted 07 March 2018 Abstract : Selenium, a non-metal, is a member of chalcogen group which is a naturally occurring metalloid. It exists in +6, +4, 0 and –2 oxidation states. The permissible limit of selenium for drinking water is 0.05 mg/L (WHO guidelines) whereas; according to Bureau of Indian Standards the acceptable limit is 0.01 mg/ L. The raw organic waste material (Sweet lime ‘ Citrus limetta’ juice residue) was used for adsorbent prepa- ration for selenium removal from drinking water. The adsorbent prepared was characterized using SEM, EDS, FTIR and ZPC (Zero point charge). Batch adsorption process was used for optimization of adsorption pa- rameters. Optimization study revealed maximum removal efficiency of 93.7% for selenium concentration of 0.1 mg/L for adsorbent dose of 0.05 g/50 ml Se solution having a contact time of 30 min, agitation speed : 650 RPM, pH : 4 and temperature : 313 K. After adsorption the EDS data showed the presence of sele- nium. The Temkin isotherm is the best fitted isotherm model indicating progressive saturation of the solid and also it indicated chemical adsorption. Keywords : Selenium, batch adsorption, sweet lime juice residue, kinetics, isotherms. I. Introduction Selenium is quintessential at low concentrations but malignant at high concentrations. It is a basic nutrient for human and animal health (0.8–1.7 mol/ L), but can be lethal above 1.7 mol/L. Thermal power plants, ore-smelting plants and oil refineries liberate selenium in the form of selenite and sel- enate. Considerable amounts of soluble selenium are present in industrial effluents. The main physico- chemical methods used mainly for removal of sele- nium are precipitation by chemicals, ion-exchange and by catalytic reduction. The chemical methods are costly 1 . The permissible limit of selenium for drinking water is 0.05 mg/L 2 whereas; according to Bureau of Indian Standards the acceptable limit is 0.01 mg/L. It exists in oxidation states of +6, +4, 0 and –2 as selenides (Se 2– ), elemental selenium (Se 0 ), selenites (Se 4+ ) and selenates (Se 6+ ) 2 . The oxida- tion states are the major feature of selenium chemis- try that affects the movement and solubility of sele- nium in water. +6 oxidation state (selenates) are more soluble in water. According to EPA, finger nail changes, damage to the peripheral nervous sys- tem, irritability and fatigue are caused by short-term exposure to selenium whereas long term or life time exposure exceeding MCL (Maximum Contaminant Level is 0.05 mg/L and 0.04 mg/L enforced by EPA and WHO respectively) causes finger nail and hair loss, dermatitis, damage to liver and kidney tissues and, the nervous and circulatory systems. By con- suming selenium accumulator plant the syndromes “blind staggers” and “alkali disease” are being found in livestock. After chronic exposure to both inor- ganic and organic compounds of selenium, neuro- toxicity (deterioration of motor neurons) might oc- cur, leading to greater risk of occurrence of amyo- trophic lateral-sclerosis. The long-term exposure to environmental selenium is difficult to assess for morbidity and toxicity 3 . The selenium can be re- moved by physical, chemical or bio treatment. Sev- eral technologies are used for removal of selenium at municipality level using activated-alumina adsorp- tion, distillation, ion-exchange, reverse osmosis, co- agulation and filtration, lime softening. Media filtra-