Sorption Properties of Surface Modified Activated Carbon and Polymer Hydrogels for Environmental Remediation

Abstract

In this work, the removal of eight ionic liquids (ILs) of types of bromide based imidazolium, pyrrolidinium and pyridinium having different alkyl chain lengths, two dyes (nylosan red N-2RBL, palatine orange) and copper from simulated waste water were investigated. Three adsorbent systems were used depending upon the type of adsorbate. These systems include inorganic system based on activated carbon (AC) and polymer based system including both natural and synthetic polymer. Two types of activated carbons (ACs), fabric and granulated, were used. The granulated activated carbon was further modified using nitric acid and sodium hypochlorite as oxidizing agents, to enhance the oxygenated functional groups on AC. They were well characterized in terms of surface chemistry by “Boehm” titrations and pH of point of zero charge measurements and porosity by N2 adsorption at 77 K and CO2 adsorption at 273 K. Upon modification, AC contained carboxylic, lectonic, and phenolic type’s functionality. The adsorptions of ILs on these ACs were studied at different temperatures (25-55 °C) and pH range. Thermodynamic studies indicated that the adsorption of ILs onto ACs was an exothermic process. Their removal efficiency increased with increase in alkyl chain length, which was due to the increase in hydrophobicity of long chain ILs cations determined with the evolution of the calculated octanol–water constant (Kow) and negative values of free energies indicated its spontaneous nature. The 2nd adsorption system used in this work was based on polymer membranes prepared from chitosan (CS). The chitosan was mixed with two different amount of poly vinyl alcohol (PVA) and chemically crosslinked by using a new crosslinking agent i.e. methyltrimethoxysilane (MTMS). The crosslinked CS/PVA membranes showed hydrogel properties and swelling was decreased with increase in PVA content. Infrared spectroscopy confirmed the crosslinking reaction between the feed components and the existence of siloxane bond. The membrane swelling was greatly affected by pH, ionic strength and temperature of the solution. These membranes showed high swelling in acidic and low swelling in basic pH range. This switchable pH response of these membranes was exploited and used to adsorb dyes from aqueous solution. The effect of dye concentration, contact time, adsorbent amount and pH on the selectivity and sensitivity of the removal process was investigated. The pH of solution greatly affected the removal efficiency and maximum adsorption was observed at pH 3. Thermodynamic parameters suggested that the dyes adsorption on the membrane was spontaneous and the process was endothermic. The effect of time, pH and salt concentration on swelling were investigated. The high adsorption of dyes in acidic media is very useful because most of textile effluents in acidic pH range and the membranes are quite suitable for such type of system. In 3rd system, acrylic acid was polymerized by gamma radiations in the presence of phenyltriethoxysilane (PTES). Different amounts of PTES were incorporated in acrylic acid and irradiated at different doses upto maximum of 30 kGy. The crosslinked poly (acrylic acid) (PAA) showed hydrogel properties and adsorb maximum of 246 g.g-1 of water. The increased PTES concentration decreased the EDS of the PAA hydrogels. Thermogravimetric analysis showed an increase in the stability of the hydrogels having high PTES content. The swelling of the hydrogel affected by pH, ionic strength and temperature. These hydrogels showed low swelling in acidic and basic pH range and high swelling around neutral pH. The adsorption of copper onto these hydrogels was studied. The pH of solution greatly affected the removal efficiency and maximum adsorption was achieved at pH 3. The effect of contact time, dye concentration, adsorbent amount and pH on the selectivity and sensitivity of the removal process was investigated. All the data of ionic liquids, dyes and copper metal were analyzed by applying different kinetics models such as: pseudo-first order and pseudo-second order, models, diffusion law and Boyd law. The equilibrium adsorption capacities of the adsorbent for all adsorbates removal were measured and the experimental data was analyzed by applying adsorption model such as: Langmuir, Freundlich and Langmuir Freundlich isotherm models.