Adsorption Behaviors And Mechanisms Of Aromatic Acids By Soil Iron Oxides

Use of reclaimed water is a good way to solve water shortages.However,polar pollutants(e.g.,aromatic acids)in reclaimed water may be introduced into environment,posing a contamination risk on groundwater.Iron oxides are ubiquitous in soils or sediments.To understand the sorption mechanisms of polar pollutants onto iron oxides is of importance for their transport risk evaluation.In this study,adsorption of model polar pollutants(aromatic acids)onto soils iron oxides was investigated by column chromatography,batch experiments,spectroscopic analysis and surface complexation model.Meanwhile,the influence of environmental factors(e.g.,solution p H,ionic strength,temperature and coated organic acids)on adsorption was also studied.The main conclusions are as follows:(1)Reversible adsorption of 32 monocarboxylic acids(i.e.,benzoic acids,naphthoic acids and acidic pharmaceuticals)onto goethite was dominated by H-bonding.In contrast,polycarboxylic acids(e.g.,phthalic acid,trimellitic acid and pyromellitic acid)can be adsorbed onto goethite by inner-sphere complexation,which is hard to desorb.Bonding of monocarboxylic acids was endothermic(?H > 0)while adsorption of monophenols and nonpolar organic compounds was exothermic(?H < 0),indicating the different bonding strength.Polyparameter linear free energy relationship models were established to provide acceptable fitting results of the goethite-solute distribution coefficients.(2)At p H 3.4-7.5,bonding of three non-steroidal anti-inflammatory drugs(ketoprofen,naproxen and diclofenac)onto goethite was totally reversible,ionic strength-dependent and endothermic(adsorption enthalpy 2.86-9.75 k J/mol).These evidences supported H-bonding mechanism,which was further explained by ATR-FTIR observation and a triple planes model.Surface-bound organic acids(phthalic acid,trimellitic acid and pyromellitic acid)by inner-sphere complexation with goethite changed hydrophobicity and charge of the surface.Thus,it influenced the adsorption of three pharmaceuticals by affecting the H-bonding,electrostatic interaction and van der Waals interactions.(3)Iron oxides(e.g.,goethite and hematite)had greater adsorption affinity for diclofenac than kaolinite and aluminium oxide because of the more effective adsorption sites(surface-OH groups).Adsorption of diclofenac onto soils with low soil organic matter(foc: 0.61-2.04%)was dominated by iron oxides rather than soil organic matter.The contents of Fe extracted by DTPA method rather than that of the total iron oxides can be used to predict the adsorption coefficients of diclofenac,suggesting that bonding was adsorption site-dependent.