Sorption studies of 4-NP onto goethite: effects of contact time, pH, concentration, ionic strength and temperature

Abstract

The effects of different state variables (contact time, pH, concentration, ionic strength and temperature) were investigated on the adsorption of 4-nitrophenol (4-NP) onto goethite for its potential utilization in mitigating water pollution. The results suggested that the adsorption of 4-NP reached a quasi-steady state equilibrium at 24 h and followed pseudo-second order kinetics. The computed value of pseudo-second order rate coefficient for adsorption of 4-NP on goethite was 447.57 kg mM−1 h−1. Adsorption of 4-NP on goethite was higher when the solution pH was < pHzpc of goethite and near pKa(7.15)of 4-NP and vice versa. The effect of concentration of 4-NP on adsorption revealed the heterogeneity of sorption sites on goethite surface; highly selective sites of higher bonding energy up to sorption density of 1.42 mM kg−1 and lower selectivity sites of lesser bonding energy for sorption density above 2.07 mM kg−1. An increase in ionic strength, increased the adsorption of 4-NP on goethite but an increase in temperature decreased the adsorption. The changes in thermodynamic parameters indicated that adsorption of 4-NP was a spontaneous and exothermic reaction with an increase in the entropy of the system. Based on these results, the optimal conditions for the maximal removal of 4-NP were 6.7–7.0 pH, 0.015–0.02 ionic strength, 15 °C temperature with 1: 100 goethite (sorbent)-solution ratio.