The Adsorption Mechanisms Of Carboxylic Acid Compounds At Clay-water Interface

Kaolinite is a major type of clay mineral in the soils and sediments in warm climates. Carboxylic acid compounds are often found in the environment. The sorption of carboxylic acid compounds onto kaolinite is very important in soil-water environmental. In this research, sorption of one aromatic carboxylic acid compound(naproxen, NPX) and three low molecular weight(LMW) organic acids(acetate, oxalate and citrate) onto kaolinite was investigated by batch experiments and Fourier transform Infrared spectroscopy. Main conclusions are drawn as followed:(1) Carboxyl group and aromatic ring of naproxen were all important in its sorption onto kaolinite. The primary sorption mechanism is n-π electron donor-acceptor(EDA) interaction between aromatic ring of naproxen(π-electron acceptors) and the siloxane oxygens(n-donors) of kaolinite. The carboxyl group sorbed onto the surfaces of kaolinite by hydrogen bond can contribute to the overall sorption. Naproxen can be retained by siloxane surfaces of kaolinite.(2) When solution pH increased from 3.0 to 6.0, the percentage of NPX0 decreased and hydrogen bond also became weak. As a result, the whole sorption of naproxen decreased. Based on experimental data, sorption model of naproxen was established: Kd,NPX = Kd,EDA + Kd,HB f0. f0 is the percentage of NPX0. This conceptual model is precise to assess the contribution of each sorption mechanism to the whole sorption.(3) LMW organic acid with higher hydrophobicity showed greater competition ability. The competitive ability for sorption sites of kaolinite followed the order: acetate>citrate>oxalate. Similar adsorption behavior of three LMW organic acids onto kaolinite as a function of solution pH can be observed. For each organic acid, the maximum adsorption occurred in a certain pH range, in which all the carboxylic groups of organic acids are almost dissociated and target sorption sites(≡Al OH) were abundant. This is in line with previous studies.(4) The sorption free energy change(ΔG) of LMW organic anions was influenced by hydrophilia, chemisorption, electrostatic interaction and physisorption. The adsorption of three organic anions was mainly controlled by chemisorption(ΔGchemisorption from-26.6 to-79.6 kJ/mol) and hydrophilia(ΔGhydrophilia from 22.8 to 68.2 kJ/mol). Interactions like electrostatic repulsion(ΔGelectrostatic repulsion from 2.5 to 7.7 kJ/mol) and physisorption(ΔGphysisorption from-5.4 to-11.4 kJ/mol) are very weak. With increasing carboxylic groups(-COO-), chemisorption was enhanced, but hydrophilia effect and electrostatic repulsion became stronger as well. Thus, the sorption followed the order: oxalate > citrate > acetate.(5) When solution pH = 8.0, kaolinite(pHpzc=3.8) has a negatively charged surface. The increasing ionic strength(from 0.001 to 0.1 mol/L, NaCl) caused a less negatively charged kaolinite surface. For the chemisorption of LMW organic anions, they replaced equivalent amounts of surface-OH in their complexation with kaolinite surfaces. In this case, the ζ-potential of kaolinite surface did not shift significantly. LMW organic anions could be retained in the electric double layer around mineral surface by physisorption, which caused the change of ζ-potential.