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

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.