Effect of Soil-pH, temperature and moisture content on sorption dynamics of Metformin and Erythromycin

The rising soil-groundwater quality issues due to pharmaceuticals and personal care products (PPCPs) have spurred significant concern. To understand the sorption characteristics of metformin (MTN) and erythromycin (ETM) in sandy and sandy loam soils with varying organic matter and particle composition, sorption kinetics (single and competitive), isotherms, and thermodynamics were studied. The effects of pH and soil moisture content (SMC) were also investigated at environmentally relevant concentrations. The equilibrium time of MTN and ETM sorption by the three soils in a competitive solute system was about 4 h, and the sorption process was in line with a pseudo-second-order model. The rate-determining step in the process involved both intraparticle diffusion and liquid film diffusion mechanisms for the two PPCPs. The highest pollutant uptake occurred in soils with higher organic matter, driven by enhanced H-bonding, electrostatic interactions, and π-π and n-π interactions facilitated by the organic matter. The equilibrium data in the three soils was well described by the Freundlich model and confirmed favourable adsorption (1/n_f = 1.01 - 1.90). The sorption coefficient (K_d) on the three soils ranged from 2.1 to 332 L/kg for MTN and from 6.25 to 845 L/kg for ETM. The adsorption process was feasible at 293 K and 303 K (ΔG° = - 0.16 to -10.24 kJ/mol), physical and exothermic in nature (ΔH° = -75.21 to -10.30 kJ/mol) for both the contaminants. Observed alterations in Q_e with pH confirmed the participation of electrostatic interactions. A low SMC favoured both MTN and ETM sorption onto the sandy soil. Overall, ETM exhibits higher expected sorption, whereas MTN has a greater tendency for migration in the soils and is thus liable to contaminate the groundwater. The study accentuates novel insights into the transport and fate of MTN and ETM in soil-groundwater systems at environmentally relevant concentrations.