Adsorption of Chromium (III) on Melamine: Kinetic, Isotherm, Thermodynamics and Mechanism Analysis

Investigation of melamine and cyanuric acid concentration in several brands of liquid milk and its non-carcinogenic risk assessment in adults and infants

In this study, the melamine and cyanuric acid concentration of widely used milk brands and the probability of non-carcinogenic risk of the brands for adults and infants were investigated. These values were 1.37 mg/L, 1.10 mg/L, and 1.09 mg/L, which corresponded to creamy sterilized sample, high-fat (creamy) pasteurized sample, and low-fat (less-creamy) pasteurized sample, respectively. Similarly, the highest amount of cyanuric acid occurred in brand A with the values of 0.79 mg/L, 0.65 mg/L, and 0.64 mg/L, which was reported in the same samples mentioned for melamine. The HQ (Hazard Quotient) of melamine in the brands of A, B, C and D for adults was 0.0025, 0.0011, 0.0006 and 0.0008 respectively. These values for infants were reported as 2.2280, 0.9444, 0.5714 and 0.6714 respectively. The risk probability of melamine for adults was less than 1. However, the HQ in brand A for infants was greater than 1 (2.380), which indicate the high probability of non-carcinogenic risk. Furthermore, the HI (Hazard Index) values of the brands of A, B, C and D for infants were 2.7913, 1.1737, 0.7067 and 0.838, respectively. The simultaneous melamine and cyanuric acid in the brands A and B in for infants increase the non-carcinogenic risk probability by approximately 2.8 and 1.2 times, respectively. The results revealed that the melamine and cyanuric acid concentrations in creamy milk samples (0.5%) were higher than in less-creamy milk samples (2.5%). Moreover, the amount of the compounds in sterilized milk samples was higher than pasteurized. In this study, a conversion factor (0.7) was proposed in order to find out the concentration of cyanuric acid in milk sample with the amount of melamine is known but the cyanuric acid concentration is unknown.

Highly efficient oxidative-alkaline-leaching process of vanadium-chromium reducing residue and parameters optimization by response surface methodology

Vanadium-chromium reducing residue was not only a typical solid waste in the steel industry but also a valuable secondary source for recovery of vanadium and chromium. A highly efficient oxidative-alkaline-leaching technology with Na₂S₂O₈ was applied in this work. The effect of experimental factors including m(NaOH)/m(Residue), liquid-to-solid ratio, reaction temperature, m(Na₂S₂O₈)/m(Residue) and reaction time, on the leaching process were investigated. It was showed that 96.3% vanadium was leached out under selected conditions: m(NaOH)/m(Residue) = 0.30, liquid-to-solid ratio of 5 mL/g, reaction time of 60 min, m(Na₂S₂O₈)/m(Residue) = 0.50, reaction temperature of 90°C and stirring rate at 500 rpm, respectively. The leaching kinetics behaviour analysis demonstrated that the controlling step of the reaction was the diffusion of residue through the liquid film, and the Ea for vanadium leaching out was calculated to 15.57 kJ/mol. Response surface methodology was applied to analyze the interaction of the main conditions and the results showed that the influence of experimental factors on the leaching efficiency of vanadium followed the order: m(NaOH)/m(Residue) (B) > m(Na₂S₂O₈)/m(Residue) (C) > reaction temperature (E) > reaction time (D) > liquid-to-solid ratio (A).

Sustainable and efficient technologies for removal and recovery of toxic and valuable metals from wastewater: Recent progress, challenges, and future perspectives

Due to their numerous effects on human health and the natural environment, water contamination with heavy metals and metalloids, caused by their extensive use in various technologies and industrial applications, continues to be a huge ecological issue that needs to be urgently tackled. Additionally, within the circular economy management framework, the recovery and recycling of metals-based waste as high value-added products (VAPs) is of great interest, owing to their high cost and the continuous depletion of their reserves and natural sources. This paper reviews the state-of-the-art technologies developed for the removal and recovery of metal pollutants from wastewater by providing an in-depth understanding of their remediation mechanisms, while analyzing and critically discussing the recent key advances regarding these treatment methods, their practical implementation and integration, as well as evaluating their advantages and remaining limitations. Herein, various treatment techniques are covered, including adsorption, reduction/oxidation, ion exchange, membrane separation technologies, solvents extraction, chemical precipitation/co-precipitation, coagulation-flocculation, flotation, and bioremediation. A particular emphasis is placed on full recovery of the captured metal pollutants in various reusable forms as metal-based VAPs, mainly as solid precipitates, which is a powerful tool that offers substantial enhancement of the remediation processes' sustainability and cost-effectiveness. At the end, we have identified some prospective research directions for future work on this topic, while presenting some recommendations that can promote sustainability and economic feasibility of the existing treatment technologies.

Thermodynamic and Kinetic Studies on Adsorption of Vanadium with Glutamic Acid

Many hydrometallurgy methods, including chemical precipitation, ion exchange, solvent extraction, and adsorption, have been used to recover vanadium from vanadium solution, but the final step of these methods involved precipitation with ammonium salts, high concentrations of which are harmful to the environment. The key point is to find a new compound to replace ammonium salts without reducing the vanadium precipitation efficiency. The adsorption process of vanadium with glutamic acid is investigated. The effects of experimental factors, including dosage of glutamic acid, reaction temperature, concentration of H2SO4, and reaction time, on the adsorption process are investigated. The results show that nearly 91.66% vanadium is adsorbed under the following reaction conditions: reaction temperature of 90 °C, H2SO4 concentration of 20 g/L, glutamic acid dosage at n(glu)/n(V) = 3.0:1, and reaction time of 60 min. The response surface methodology is applied to optimize the reaction conditions. The analysis results indicate that the reaction temperature has the greatest effect on the adsorption efficiency of vanadium and the influence of experimental factors follows the order: reaction temperature > dosage of glutamic acid to vanadium > reaction time > concentration of H2SO4. The pseudo-second-order model is selected to describe well the adsorption kinetic behavior, and the thermodynamic analysis results indicate that the adsorption process of vanadium is unspontaneous and exothermic. The results will be useful for further applications of glutamic acid, and they provide a bright future for vanadium recovery.

Efficient Removal of Cr (VI) with Biochar and Optimized Parameters by Response Surface Methodology

A highly efficient reduction process of Cr (VI) with biochar was conducted in this paper. The results showed that nearly 100% Cr (VI) was reduced at selected reaction conditions: Dosage of biochar at m (C)/m(Cr) = 3.0, reaction temperature of 90 °C, reaction time of60 min, and concentration of H2SO4 of 20 g/L. The reduction kinetics analysis demonstrated that the reduction of Cr (VI) fitted well with the pseudo-first-order model and the apparent activation energy was calculated to be 40.24 kJ/mol. Response surface methodology confirmed that all of the experimental parameters had a positive effect on the reduction of Cr (VI). The influence of each parameter on the reduction process followed the order: Dosage of biochar>concentration of H2SO4>reaction temperature >reaction time. This paper provides a versatile strategy for the treatment of wastewater containing Cr (VI) and shows a bright tomorrow for wastewater treatment.

Adsorption behavior of Fe (III) in aqueous solution on melamine

This paper focused on the adsorption behavior of Fe (III) in aqueous solution on melamine. The effects of experimental conditions including dosage of melamine, reaction time and reaction temperature were investigated. The results showed that nearly 99% Fe (III) was adsorbed under the optimal conditions: melamine dosage (mole ratio) at n(C₃H₆N₆)/n(Fe) = 3.5:1, reaction time of 60 min and reaction temperature of 90 °C. The optimal processing factors were obtained from response surface methodology and the effects of processing parameters on the removal efficiency of Fe (III) followed the order: mole ratio (n(C₃N₆H₆):n(Fe)) > reaction temperature > reaction time. The adsorption kinetics behavior was fitted well with the pseudo-second-order model. The thermodynamic study showed that the adsorption process was unspontaneous and endothermic. The value of free energy change and standard enthalpy change disclosed that the mechanism of adsorption onto melamine was physisorption. The results will be useful for further applications of system design in the treatment of practical waste effluents.

Reduction behavior of chromium(VI) with oxalic acid in aqueous solution

The direct Cr(VI) reduction process by oxalic acid was conducted. The existence of Cr(VI) in the reaction medium was measured by software Visual MINTEQ and the concentration of Cr(VI) was measured by ICP-OES. The results showed that the Cr(VI) was efficiently reduced by oxalic acid at high reaction temperature and high dosage of oxalic acid. The reduced product, Cr(III), was easily generated stable complex compounds (Cr(HC2O4)3) with oxalate, which displayed a negative effect on the reduction process. The high reaction temperature and high acidic medium could destroy the stable structure of a complex compound to release oxalate, and facilitate the reduction of Cr(VI). Generally, the results showed in this paper provided a versatile strategy for Cr(VI) reduction and exhibited a bright application future for real wastewater treatment.