Removal of Inorganic Anionic Pollutants from Water using Adsorption Technology

Sammanfattning: In the era of urbanization, industrialization and population growth, groundwater and drinking water sources are getting adversely polluted due to the addition of different toxic contaminants including inorganic anionic pollutants. The inorganic anions are of serious concern due to their adverse health effects on humans, even when present at very low concentrations in water. Adsorption process is an attractive method for the removal of anions as compared to other water treatment technologies in terms of cost, simplicity of design and operation. In this study, granular ferric hydroxide (GFH) and nano-Al2O3 were tested for the removal of fluoride, perchlorate and nitrate anions from aqueous solutions. Different experimental parameters (viz. pH, agitation time, adsorbate concentration, temperature, competing anions) have been studied to optimize the adsorption process. The maximum adsorption capacity of 7.0 mg g-1 (at pH 6.0-7.0) and 20.0 mg g-1 (at pH 6.0-6.5) for fluoride and perchlorate, respectively was achieved using GFH at 25 oC. Adsorption kinetics of fluoride by GFH was favorably explained with pseudo-first-order, while perchlorate adsorption kinetics followed pseudo-second-order model. The Langmuir model explained the adsorption isotherms of fluoride and perchlorate by GFH. The Raman spectroscopy results revealed that perchlorate was adsorbed through electrostatic attraction between perchlorate and positively charged GFH surface sites. The adsorption efficiencies achieved by nano-Al2O3 for nitrate and fluoride were 4.0 mg g-1 (at pH ~4.4) and 14.0 mg g-1 (at pH ~6.15), respectively at 25 oC. Kinetics and isotherms of fluoride and nitrate by nano-Al2O3 were well-explained by pseudo-second-order model and Langmuir isotherm model, respectively. The FTIR and EDX results reveal that aluminum-fluoro complexes are formed due to the interaction between fluoride and nano-Al2O3 moieties. In all the cases, the most influencing anions were the ones that compete for similar binding sites on the adsorbent surface. Results from this study will be helpful in demonstrating potential utility of the tested adsorbents for the removal of different anions from water and provide an insight into the adsorbent-adsorbate (anions) interactions in the aqueous media.