Characteristics of phosphate adsorption on ferric hydroxide synthesized from a Fe2(SO4)3 aqueous solution discharged from a hydrometallurgical process

Quantum parameter analysis of the adsorption mechanism by freshly formed ferric hydroxide for synthetic dye and antibiotic wastewaters

In this study, the adsorption effect by freshly formed ferric hydroxide (FFFH) for the removal of 47 synthetic dye and antibiotic wastewaters under different pH conditions (i.e., pH = 4, 7, and 10) was investigated. The average total organic carbon (TOC) removal rates (R_exp) of pollutants under acidic, neutral, and alkaline conditions were 27.10 ± 3.21%, 15.16 ± 2.48%, and 9.72 ± 2.81%, respectively. By analyzing the characteristics of FFFH measured by SEM, XRD, FT-IR, TGA and BET, the properties of pollutants, and the values of R_exp, one can conclude that the large specific surface area and rich hydroxyl groups on the surface of FFFH were the reasons for its adsorption capacity for organic pollutants, and the electrostatic adsorption was the main reason for higher removal rate in acidic condition. Subsequently, to better elucidate the intrinsic factors influencing the removal rates at the molecular structure level, three optimal quantitative structure-activity relationship (QSAR) models were established by using multiple linear regression (MLR) analysis. Results of model validations (e.g., regression coefficient, internal and external verifications, and Y-randomization) showed that the established models exhibited excellent stability, reliability, and robustness with the values of R² = 0.7544, 0.7764, 0.7528, Q²_INT = 0.6451, 0.6836, 0.6228, and Q²_EXT = 0.5890, 0.6029, 0.7298 under acidic, neutral, and alkaline conditions, respectively. The results of quantum parameter analysis revealed that the adsorption mechanism of FFFH for dyes and antibiotics mainly includes the activity of adsorption site, the behavior of electron transfer and the strength of molecular polarity.

Evaluation of phosphate removal from aqueous solution using metal organic framework; isotherm, kinetic and thermodynamic study

BACKGROUND

Phosphate (PO₄ ^3-) is the main etiological factor of eutrophication in surface waters. Metal organic frameworks (MOFs) are novel hybrid materials with amazing structural properties that make them a prominent material for adsorption.

METHODS

Zeolitic imidazolate framework 67 (ZIF-67), a water stable member of MOFs, with a truncated rhombic dodecahedron crystalline structure was synthesized in aqueous environment at room temperature and then characterized using XRD and SEM. PO₄ ^3- adsorption from synthetic solutions using ZIF-67 in batch mode were evaluated and a polynomial model (R²: 0.99, R² _adj: 0.98, LOF: 0.1433) developed using response surface methodology (RSM).

RESULTS

The highest PO₄ ^3- removal (99.2%) after model optimization obtained when ZIF-67 dose, pH and mixing time adjusted to 6.82, 832.4 mg/L and 39.95 min, respectively. The optimum PO₄ ^3- concentration in which highest PO₄ ^3- removal and lowest adsorbent utilization occurs, observed at 30 mg/L. PO₄ ^3- removal eclipsed significantly in the presence of carbonate. The equilibrium and kinetic models showed that PO₄ ^3- adsorbed in monolayer (q_max: 92.43 mg/g) and the sorption process controlled in the sorption stage. Adsorption was also more favorable at higher PO₄ ^3- concentration, according to the separation factor (K_R) graph. Thermodynamic parameters (minus signs of ∆G°, ∆H° of 0.179 KJ/mol and ∆S° of 44.91 KJ/mol.K) demonstrate the spontaneous, endothermic and physisorption nature of the process.

CONCLUSION

High adsorption capacity and adsorption rates, make ZIF-67 a promising adsorbent for PO₄ ^3- removal from aqueous environment.

Synthesis of mesoporous lanthanum hydroxide with enhanced adsorption performance for phosphate removal

Phosphate is a ubiquitous pollutant in aquatic systems, and increasingly stringent post-treatment phosphate effluent standards necessitate increasingly efficient removal techniques. In this study, mesoporous lanthanum hydroxide (MLHO) was synthesized by a hard-template method using ordered mesoporous silica, and its potential as an adsorbent for high-efficiency phosphate removal in aqueous solutions was tested. The porosity characteristics of MLHOs were controlled by adjusting the template structure and synthesis conditions. MLHO adsorbents showed great potential for phosphate removal from solutions containing both high and low initial phosphate concentrations. The phosphate adsorption capacity of MLHO strongly depended on its surface area as this process was governed by monolayer adsorption. Moreover, the phosphate removal performance of MLHO was affected by its structural properties. MLHO showed a high adsorption capacity of 109.41 mg P g⁻¹ at 28 °C (q_m by the Langmuir isotherm model). Further, it showed ultrafast adsorption in a solution with low initial concentration of 2 mg P/L; within the first 10 min, 99.8% of phosphate was removed, and the phosphorus concentration remaining in the solution dramatically reduced to 4 μg P/L. These findings suggest that MLHO adsorbent is a good candidate for rapid and efficient low-concentration phosphate removal to meet the increasingly stringent discharge standards for wastewater treatment plants.

MLHO exhibited outstanding phosphate removal performance with a high adsorption capacity as well as efficient removal in low P concentrations.

Ferrihydrite-impregnated granular activated carbon (FH@GAC) for efficient phosphorus removal from wastewater secondary effluent

Adsorptive removal of phosphorus from wastewater effluents has attracted attention because of its reduced sludge production and potential P recovery. In this study, we investigated granular activated carbons (GACs) impregnated with amorphous ferrihydrite (FH@GAC) for the sorption of phosphorus from aqueous solutions. Preoxidation of intact GAC surfaces using an oxidant (e.g., hypochlorite) and strong acids (e.g., HNO₃/H₂SO₄) was performed to create active functional groups (e.g., carboxyl or phenolic) for enhanced iron binding, leading to greater phosphorus uptake. Both the rate and the capacity of phosphorus sorption onto FH@GAC had significant, positive relationships (Pearson correlation coefficient r > 0.9) with the product of surface area and Fe content. The pseudo-second-order reaction kinetics explained the P sorption rate better than the pseudo-first-order reaction kinetics, whereas the Langmuir model fit the P sorption isotherm better than the Freundlich model. The iron content in the FH@GAC increased significantly (>10 mg/g) when GAC (e.g., BMC1050) was preoxidized by a 1:1 (w/w) concentrated HNO₃/H₂SO₄ mixture. The Langmuir maximum P sorption capacity of a functionalized FH@BMC1050 adsorbent prepared with acid pretreatment was estimated to be substantial (5.73 mg P/g GAC corresponding to 526 mg P/g Fe). This sorption capacity was superior to that of a FH slurry, possibly because the nano-sized FH formed inside the GAC pores (<2.5 nm) can bind phosphate ions more effectively than FH aggregates. Fixed-bed column reactor operation with bicarbonate regeneration showed potential for efficient, continuous phosphorus removal by FH@GAC media.