Removal of nitrate by synthetic organosilicas and organoclay: Kinetic and isotherm studies

Polymer-Clay Nanocomposites for the Uptake of Hazardous Anions

Polymer intercalated clay nanocomposites were prepared from various montmorillonites (Mt) and a polymer, polydiallyldimethylammonim (PDDA) chloride. X-ray diffraction (XRD) analysis of the above polymer intercalated nanocomposites showed either no crystalline peaks or very broad peaks with the intercalation of PDDA polymer in the interlayers, probably as a result of exfoliation of the clay layers. Infrared spectroscopy revealed the presence of PDDA in all the clay nanocomposite materials. The maximum adsorption capacities of nitrate, perchlorate, and chromate by one of the polymer intercalated nanocomposite materials prepared from montmorillonite, Kunipea were 0.40 mmol·g-1, 0.44 mmol·g-1 and 0.299 mmol·g-1, respectively. The other two polymer intercalated nanocomposites prepared with montmorillonites from Wyoming and China showed very good adsorption capacities for perchlorate but somewhat lower uptake capacities for chromate and nitrate compared to the nanocomposite prepared from montmorillonite from Kunipea. The uptake of nitrate, perchlorate and chromate by the polymer intercalated nanocomposites could be well described using the Freundlich isotherm while their uptake kinetics fitted well to the pseudo-second-order model. The uptake kinetics of nitrate, perchlorate, and chromate were found to be fast as equilibrium was reached within 4 h. Moreover, the uptakes of chromate by polymer intercalated nanocomposites were found to be highly selective in the presence of Cl-, SO42- and CO32-, the most abundant naturally occurring anions.

ALLODUST augmented activated sludge single batch anaerobic reactor (AS-SBAnR) for high concentration nitrate removal from agricultural wastewater

Nitrate is among the most widespread contaminants that threaten water bodies and waterways. Under favourable environmental conditions, high nitrate concentrations in water can contribute to eutrophication, thus presenting a high potential for risk to ecosystems and human health. Low-cost allophanic soil material and carbon-based bio-wastes have great potential to reduce nutrient concentrations from contaminated waters. This study investigated the mechanisms that underpin the reduction of nitrate concentrations and nitrous oxide (N₂O) emission in the presence of novel developed media in an activated sludge process. A new operating approach, employing a newly developed media (ALLODUST), was evaluated for enhanced NO^-₃-N removal from agricultural wastewater. Two anaerobic-aerobic batch reactors were developed, where the coupled bottom aeration method was used for efficient agitation and aeration in the aerobic reactor. The reactor was run at high NO^-₃-N concentrations (110 mg L^-1), under anoxic conditions at low- to long-term contact times (2, 12, and 22 h), while the aerobic period (clarification) was constant for all the experimental designs (2 h). ALLODUST retained its integrity and stability over the long-term operation. Low ALLODUST concentrations (5.95 g L^-1) removed 87% of the NO^-₃-N from the wastewater within 12 h. Further exploration revealed that the same amount of the media was optimal for decreasing N₂O emissions from the anaerobic activated sludge reactor by 80%.

Experimental and Theoretical Studies of Methyl Orange Uptake by Mn-Rich Synthetic Mica: Insights into Manganese Role in Adsorption and Selectivity

Manganese–containing mica (Mn–mica) was synthesized at 200 °C/96 h using Mn–carbonate, Al–nitrate, silicic acid, and high KOH concentration under hydrothermal conditions. Mn–mica was characterized and tested as a new adsorbent for the removal of methyl orange (MO) dye from aqueous solutions. Compared to naturally occurring mica, the Mn–mica with manganese in the octahedral sheet resulted in enhanced MO uptake by four times at pH 3.0 and 25 °C. The pseudo–second order equation for kinetics and Freundlich equation for adsorption isotherm fitted well to the experimental data at all adsorption temperatures (i.e., 25, 40 and 55 °C). The decrease of Langmuir uptake capacity from 107.3 to 92.76 mg·g⁻¹ within the temperature range of 25–55 °C suggested that MO adsorption is an exothermic process. The role of manganese in MO selectivity and the adsorption mechanism was analyzed via the physicochemical parameters of a multilayer adsorption model. The aggregated number of MO ions per Mn–mica active site (n) was superior to unity at all temperatures signifying a vertical geometry and a mechanism of multi–interactions. The active sites number (D_M) of Mn–mica and the total removed MO layers (N_t) slightly changed with temperature. The decrease in the MO adsorption capacities (Q_sat = n·D_M·N_t) from 190.44 to 140.33 mg·g⁻¹ in the temperature range of 25–55 °C was mainly controlled by the n parameter. The results of adsorption energies revealed that MO uptake was an exothermic (i.e., negative ΔE values) and a physisorption process (ΔE < 40 kJ mol ⁻¹). Accordingly, the adsorption of MO onto Mn–mica was governed by the number of active sites and the adsorption energy. This study offers insights into the manganese control of the interactions between MO ions and Mn–mica active sites.

A novel nanocomposite of aminated silica nanotube (MWCNT/Si/NH2) and its potential on adsorption of nitrite

Several parts of the world have been facing the problem of nitrite and nitrate contamination in ground and surface water. The acute toxicity of nitrite has been shown to be 10-fold higher than that of nitrate. In the present study, aminated silica carbon nanotube (ASCNT) was synthesised and tested for nitrite removal. The synergistic effects rendered by both amine and silica in ASCNT have significantly improved the nitrite removal efficiency. The IEP increased from 2.91 for pristine carbon nanotube (CNT) to 8.15 for ASCNT, and the surface area also increased from 178.86 to 548.21 m² g^-1. These properties have promoted ASCNT a novel adsorbent to remove nitrite. At optimum conditions of 700 ppm of nitrite concentration at pH 7 and 5 h of contact with 15 mg of adsorbent, the ASCNT achieved the maximal loading capacity of 396 mg/g (85% nitrite removal). The removal data of nitrite onto ASCNT fitted the Langmuir isotherm model better than the Freundlich isotherm model with the highest regression value of 0.98415, and also, the nonlinear analysis of kinetics data showed that the removal of nitrite followed pseudo-second-order kinetic. The positive values of both ΔS° and ΔH° suggested an endothermic reaction and an increase in randomness at the solid-liquid interface. The negative ΔG° values indicated a spontaneous adsorption process. The ASCNT was characterised using FESEM-EDX and FTIR, and the results obtained confirmed the removal of nitrite. Based on the findings, ASCNT can be considered as a novel and promising candidate for the removal of nitrite ions from wastewater.

Kinetic and isotherm studies of nitrate adsorption on granular Fe–Zr–chitosan complex and electrochemical reduction of nitrate from the spent regenerant solution

In this study, a granular Fe–Zr–chitosan complex was prepared to remove nitrate from aqueous solution and an undivided cylindrical electrochemical cell was constructed to treat the spent regenerant solution, thus achieving separation and conversion.