Nonequilibrium sorption of phenols onto geosorbents: the impact of pH on intraparticle mass transfer

A framework for assessing the retardation of organic molecules in groundwater: Implications of the species distribution for the sorption-influenced transport

The pH-dependent molecule speciation (charge state) in solution strongly influences the transport of ionizable organic compounds in the aquatic environment. Therefore, the sorption behavior is complex and reliable predictions only based on physico-chemical sorbate, sorbent and solution properties are challenging. A short overview of underlying sorption processes causing retardation during the solute transport in aquifers is completed by a description of approaches for estimating respective sorption coefficients/retardation factors and discussed together with their limitations. Based on these initial considerations, a systematic framework is proposed, which allows the assessment of transport properties of organic molecule species by their chemical nature (neutral, acidic, basic, ampholytic). As a result, the transport properties of many (ionizable) organic molecules of interest can be assessed and even first presumptions for the sorption behavior of new and not yet investigated molecules can be derived.

A combined PHREEQC-2/parallel fracture model for the simulation of laminar/non-laminar flow and contaminant transport with reactions

A combination of a parallel fracture model with the PHREEQC-2 geochemical model was developed to simulate sequential flow and chemical transport with reactions in fractured media where both laminar and turbulent flows occur. The integration of non-laminar flow resistances in one model produced relevant effects on water flow velocities, thus improving model prediction capabilities on contaminant transport. The proposed conceptual model consists of 3D rock-blocks, separated by horizontal bedding plane fractures with variable apertures. Particle tracking solved the transport equations for conservative compounds and provided input for PHREEQC-2. For each cluster of contaminant pathways, PHREEQC-2 determined the concentration for mass-transfer, sorption/desorption, ion exchange, mineral dissolution/precipitation and biodegradation, under kinetically controlled reactive processes of equilibrated chemical species. Field tests have been performed for the code verification. As an example, the combined model has been applied to a contaminated fractured aquifer of southern Italy in order to simulate the phenol transport. The code correctly fitted the field available data and also predicted a possible rapid depletion of phenols as a result of an increased biodegradation rate induced by a simulated artificial injection of nitrates, upgradient to the sources.

Sorption of p-nitrophenol onto sediment in the presence of cetylpyridinium chloride and Pb(NO3)2: influence of pH

pH and the presence of compounds have a great effect on the sorption of organic contaminants. In this study, batch equilibrium experiments were carried out to investigate the influence of pH on the sorption of p-nitrophenol (PNP) onto sediment in the presence of Pb(NO3)2 and cetylpyridinium chloride (CPC) cationic surfactant. Results indicated that in the multi-solute system with PNP, Pb(NO3)2 and CPC, the sorption of PNP increased with the increasing pH and the amount of sorbed PNP at equilibrium was much higher than in the single-solute system. This can be attributed to the presence of Pb(NO3)2 and CPC. It is believed that the main reason for that was the larger sorption of CPC at higher pH allowing higher sorption of PNP. The results are believed to provide a useful insight into describing the transport and fate of PNP in natural environments.