The characteristics and mechanisms of competitive adsorption of trace metals on bacteria-associated clay mineral composites have never been studied, despite their being among the most common organic–mineral complexes in geological systems. Herein, competitive adsorption of Pb and Cd on Pseudomonas putida–montmorillonite composite was investigated through adsorption–desorption experiment, isothermal titration calorimetry (ITC), and synchrotron micro X-ray fluorescence (μ-XRF). From the experiment, stronger competition was observed on clay mineral than on bacteria–clay composite because more non-specific sites accounted for heavy metal adsorption on clay mineral surface at the studied pH 5. Both competing heavy metals tended to react with bacterial fractions in the composite, which was verified by the higher correlation of Cd (and Pb) with Zn (R2 = 0.41) elemental distribution than with Si (R2 = 0.10). ITC results showed that competitive adsorption exhibited a lower entropy change (ΔS) at the metal-sorbent interfaces compared with single-metal adsorption, revealing that Cd and Pb are bound to the same types of adsorption sites on the sorbent. The competitive effect on bacteria–clay composite was found to be helpful for a better understanding on the fixation, remobilization and subsequent migration of heavy metals in multi-metal contaminated environments.

Sorption equilibria of copper(II) ions onto palygorskite and sepiolite clay minerals were studied as a function of temperature. The experimental data were fitted to the Langmuir, Freundlich, Temkin, and R-D models to obtain the isothermal constants. van’t Hoff, Gibbs, Clausius–Clapeyron, and modified Arrhenius equations were also employed to evaluate the thermodynamic parameters involved in Cu sorption. The results showed that fibrous clay minerals exhibit enhanced Cu(II) sorption capacities at higher temperatures. Enthalpy changes (ΔH°) were found to be positive, confirming that the process of Cu(II) sorption on both palygorskite and sepiolite was endothermic. Positive values were also obtained for the entropy changes (ΔS°), which suggests increased randomness at the solid-solution interface during the sorption of Cu(II) ions on both fibrous clay minerals investigated. The free energy changes (ΔG°) were negative for all the different temperatures and initial Cu(II) concentrations tested, indicating that sorption on the minerals is spontaneous and favorable. It was, therefore, concluded that sorption of Cu(II) ions on fibrous clay minerals is entropically driven. The values of isosteric heat of sorption (∆H ₓ) decreased with increasing sorption density, which shows that the clay surface is heterogeneous in terms of the active sites available for Cu(II) retention. The values of activation energy (E ₐ) and sticking probability (S *) generally lied within the ranges associated with physisorption for palygorskite and chemisorptions for sepiolite. In conclusion, the thermodynamic parameters investigated revealed the higher tendency and capacity of sepiolite, compared to palygorskite, for the feasible, spontaneous, and endothermic retention of Cu(II). However, the intensity of Cu(II) interactions with the fibrous clay minerals was found to depend to a large extent on the temperature and the initial Cu loading of the systems.

Residual organic matters in the secondary effluent are usually less biodegradable in terms of the total organic carbon content, and when discharged into a receiving water body, their further decomposition most likely mainly occurs due to chemical oxidation. Using this scenario, a semi-empirical method was previously developed to calculate the thermodynamic entropy of organic oxidation to quantitatively evaluate the impact of organic discharge on the water environment. In this study, the relationship between the entropy increase (ΔSC) and excess organic mass (ΔTOC) was experimentally verified via combustion heat measurement using typical organic chemicals and mixtures. For individual organic chemicals, a linear relationship was detected between ΔSC and ΔTOC with the same proportionality coefficient, 54.0 kJ/g, determined in the previous semi-empirical relationship. For the organic mixtures, a linear relationship was also identified; however, the proportionality coefficient was 69.2 kJ/g, indicating an approximately 28 % increase in the oxidation heat required to decompose the same organic mass. This increase in energy can likely be attributed to the synergistic effects of hydrogen bonding, hydrophobic interactions, π–π interactions, and van der Waals interactions between functional groups of different organic compounds. Intermolecular interactions may result in 17–32 % more dissociation energy for organic mixtures compared to the organic components’ chemical structures. Because organics discharged into a water body are always a mixture of organic compounds, the proportionality coefficient obtained using organic mixtures should be adopted to modify the previously proposed semi-empirical equation.

In this study, separation and concentration of fatty acids (FA) from the synthetic food processing wastewater containing low concentration of FA (250 mg/L) were investigated using expanded graphite (EG) as a novel adsorbent at different temperatures (298~318 K). The adsorption results were further analyzed to verify adsorption mechanisms and thermodynamics of FA onto EG. Results show that the adsorption of FA onto EG was explained well by the Langmuir model with the maximum adsorption capacity up to 8.01 g FA/g EG at 298 K, and considerably affected by temperature. The adsorption kinetics fitted with pseudo-second-order kinetic model and the adsorption mechanism analysis showed that the intraparticle diffusion was not the rate-limiting step, but the coalescence of FA droplets played the significant role for novel adsorption of FA onto EG. The calculated activation energy and thermodynamic parameters such as Gibbs free energy change (ΔG⁰), enthalpy change (ΔH⁰), and entropy change (ΔS⁰) indicated that the adsorption of FA onto EG was very feasible, was highly spontaneous, occurred physically, was exothermic in nature, and was stable in aquatic environmental changes. Overall, FA can be effectively harvested and concentrated from the food processing wastewater by EG even at low concentration.