High exposure effect of the adsorption site significantly enhanced the adsorption capacity and removal rate: A case of adsorption of hexavalent chromium by quaternary ammonium polymers (QAPs)

Adsorption of cadmium from wastewater with activated carbons derived from pig fur biowaste: A comparative study of in-situ and ex-situ activation routes

The environmental challenges associated with cadmium contamination in wastewater have necessitated the development of high-performing activated carbons (ACs) for effective wastewater treatment. Adsorption capacity depends on both the surface area and the adsorption-active functional groups developed on the adsorbent's surface during activation. Proper manipulation of key process variables using the appropriate activation route produces highly efficient and economically viable ACs. This research investigates the viability of pig fur biowaste as a novel precursor for activated carbons using two distinct activation methods-in-situ and ex-situ. Using a central composite design (CCD) of the Response Surface Methodology (RSM), the study systematically examines the effects of impregnation ratio, carbonization temperature, and carbonization time on the cadmium adsorption capacities of the resulting ACs. The optimal conditions for in-situ activation were found to be 691 °C, 175.11 min, and an impregnation ratio of 1.784 g/g, resulting in a cadmium adsorption capacity of 91.57 %. For ex-situ activation, the optimal conditions were 468.8 °C, 80.81 min, and an impregnation ratio of 2.915 g/g, which achieved a higher cadmium adsorption capacity of 91.21 %. Both types of activated carbons maintained high efficiency after five regeneration cycles, indicating they are suitable for long-term applications requiring repeated regeneration. Although both methods produced ACs with comparable cadmium removal efficiency, the ex-situ activation route proved to be more economically viable due to its lower temperature and shorter processing time. This study demonstrates the potential of pig fur biowaste as a sustainable and underutilized resource for AC production and highlights the ex-situ activation route as the more cost-effective approach for producing high-performance adsorbents.

Insights into the proton-enhanced mechanism of hexavalent chromium removal by amine polymers in strong acid wastewater: Reduction of hexavalent chromium and sequestration of trivalent chromium

Adsorption is a green technology of treating heavy metal-contaminated strong acid wastewaters for the recycling of heavy metal and reuse of strong acid. Herein, three amine polymers (APs) with different alkalinities and electron donating abilities were prepared to investigate the adsorption-reduction processes of Cr(VI). It was found that the removal of Cr(VI) was controlled by the concentration of -NRH+ on the surface of APs at pH > 2, which relies on the alkalinity of APs. However, the high concentration of NRH+ significantly facilitated the adsorption of Cr(VI) on the surface of APs and accelerated the mass transfer between Cr(VI) and APs at strong acid environment (pH ≤ 2). More importantly, the reduction of Cr(VI) was enhanced at pH ≤ 2, due to the high reduction potential of Cr(VI) (E ≥ 0.437). The ratio of reduction to adsorption (α) of Cr(VI) was above 0.70, and the proportion of Cr(III) bonding on Ph-AP excessed 67.6 %. Finally, a proton-enhanced mechanism of Cr(VI) removal was verified by analyzing FTIR and XPS spectra as well as constructing DFT model. This study provides a theoretical basis for the removal of Cr(VI) in the strong acid wastewater.

Structural analysis of lignite-derived humic acid and its microscopic interactions with heavy metal ions in aqueous solution

Understanding heavy metal environmental behavior with humic acid (HA) is critical. There is currently a lack of information on the control of its structure organization on its reactivity to metals. The difference in HA structures under non-homogeneous conditions is critical for revealing its micro-interaction with heavy metals. The heterogeneity of HA was reduced using the fractionation method in this study, the chemical properties of HA fractions were analyzed using py-GC/MS, and the structural units of HA were proposed. Pb2+ was used as a probe to investigate the difference in the adsorption capacity of HA fractions. The microscopic interaction of structures with heavy metal was investigated and validated by structural units. The results show that as molecular weight increased, the oxygen content and the number of aliphatic chains decreased, but the opposite was true for aromatic and heterocyclic rings. The adsorption capacity for Pb2+ was as follows: HA-1 > HA-2 > HA-3. According to the linear analysis of the influencing factors of maximum adsorption capacity and possibility factors, the adsorption capacity was positively correlated with the contents of acid groups, carboxyl groups, phenolic hydroxyl groups, and the number of aliphatic chains. The phenolic hydroxyl group and the aliphatic-chain structure have the greatest impact. Therefore, structural differences and the number of active sites play an important role in adsorption. The binding energy of HA structural units to Pb2+ was calculated. It was found that the chain structure is easier to bind to heavy metals than aromatic rings, and the affinity of-COOH to Pb2+ is greater than that of -OH. These findings can help improve the adsorbent design.

Synchronously construction of hierarchical porous channels and cationic surface charge on lanthanum-hydrogel for rapid phosphorus removal

Phosphorus (P) removal from wastewater is critical for ecosystem operation and resource recovery. To facilitate the recycling of the used absorbents through balancing their adsorption and desorption performance on P, in this work, a novel porous magnetic La(OH)3-loaded MAPTAC/chitosan (CTS)/polyethyleneimine (PEI) ternary composite hydrogel (p-MTCH-La(OH)3) with enhanced bifunctional adsorption sites was synthesized by simultaneous dissolution of pre-embedded CaCO3 and CTS powder, followed by grafting PEI and loading La. Hierarchical porous channels promoted good dispersion of La(OH)3, bringing an excellent P adsorption capacity of 107.23 ± 4.96 mg P/g at neutral condition. PEI grafted with CTS increased the surface charge and enhanced the electrostatic attraction, which facilitated the desorption of P. The porous structure and abundant active sites also facilitated rapid adsorption with an adsorption rate constant of 0.1 g mg-1 h-1. p-MTCH-La(OH)3 maintained effective P adsorption despite co-existence with competing substances and after 5 cycles. Further mechanistic analysis indicated that La-P inner sphere complexation and LaPO4 crystalline transformation were the main pathways for P removal. However, electrostatic interactions contributed 17.5%-46.7% of the adsorption amount during the first 30 min of rapid adsorption, enabling 92.8% of the adsorbed P at this stage to be desorbed by alkaline solution. Based on the variations of adsorption and desorption capacity with adsorption time, a rapid unsaturated adsorption of 1-2 h was proposed to facilitate the recycling of the adsorbent. This study proposed a method to promote P adsorption and desorption by enhancing bifunctional adsorption sites, and proved that p-MTCH-La(OH)3 is a promising phosphate adsorbent.

Determination of 25 quaternary ammonium compounds in sludge by liquid chromatography-mass spectrometry

With the pandemic of COVID-19, the application of quaternary ammonium compounds (QACs), which can be used in SARS-CoV-2 disinfection products, has increased substantially. QACs cumulated in sewer system are ultimately deposited and enriched in sludge. QACs in the environment can adversely affect human health and the environment. In this study, a liquid chromatography-mass spectrometry method was established for the simultaneous determination of 25 QACs in sludge samples. Ultrasonic extraction and filtration of the samples was performed using a 50 mM hydrochloric acid-methanol solution. The samples were separated by liquid chromatography and detected in multiple reaction monitoring mode. The matrix effects of the sludge on the 25 QACs ranged from - 25.5% to 7.2%. All substances showed good linearity in the range of 0.5-100 ng/mL, with all determination coefficients (R2) greater than 0.999. The method detection limits (MDLs) were 9.0 ng/g for alkyltrimethylammonium chloride (ATMAC), 3.0 ng/g for benzylalkyldimethylammonium chloride (BAC), and 3.0 ng/g for dialkyldimethylammonium chloride (DADMAC). The spiked recovery rates were in the range of 74-107%, while the relative standard deviations were in the range of 0.8-20.6%. Considering its sensitivity, accuracy, and easy operation, the proposed method in this study was used to determine 22 sludge samples collected from a comprehensive wastewater treatment plant. The results showed that the concentrations of ΣATMACs, ΣBACs, and ΣDADMACs were 19.684, 3.199, and 8.344 μg/g, respectively. The main components included ATMAC-C16, ATMAC-C18, ATMAC-C20, ATMAC-C22, BAC-C12, and DADMAC-C18:C18, with concentrations exceeding 1.0 μg/g. The concentration relationships of different components in the congeners showed that some components were of similar origin.

Enhanced Chromium (VI) Adsorption onto Waste Pomegranate-Peel-Derived Biochar for Wastewater Treatment: Performance and Mechanism

Surface chemical modification allows for the rational construction of biochar with desirable structures and functionalities for environment purification. Fruit-peel-derived adsorbing material has been well studied in the adsorption of heavy-metal removal due to its abundance and non-toxicity, but its precise mechanism in removing chromium-containing pollutants remains unclear. Herein, we explored the potential application of engineered biochar prepared from fruit waste via chemical modification to remove chromium (Cr) from an aqueous solution. By synthesizing two types of agricultural residue-derived adsorbents, including pomegranate peel adsorbent (PG) and its modified product, pomegranate-peel-derived biochar (PG-B), via chemical and thermal decomposition methods, we elucidated the adsorption property of Cr(VI) on the studied materials and identified the cation retention mechanism of the adsorption process. Batch experiments and varied characterizations demonstrated that superior activity was exhibited in PG-B, which can contribute to the porous surfaces caused by pyrolysis and effective active sites resulting from alkalization. The highest Cr(VI) adsorption capacity is obtained at pH 4, a dosage of 6.25 g L^-1, and a contact time of 30 min. The maximum adsorption efficiency of 90.50% in a short period (30 min) was obtained on PG-B, while PG reached a removal performance of 78.01% at 60 min. The results from kinetic and isotherm models suggested that monolayer chemisorption dominated the adsorption process. The Langmuir maximum adsorption capacity is 16.23 mg g^-1. This study shortened the adsorption equilibrium time of pomegranate-based biosorbents and presents positive significance in designing and optimizing waste fruit-peel-derived adsorption materials for water purification.

Highly efficient removal of hexavalent chromium by magnetic Fe-C composite from reed straw and electric furnace dust waste

Reed straw and electric furnace dust (EFD) waste were used to prepare magnetic Fe-C composite (EFD&C) by co-precipitation and high-temperature activation method to remove Cr(VI) from water. The magnetic EFD&C owned a large specific surface (536.61 m²/g) and a porous structure (micropores and mesopores), and had an efficient removal capacity for Cr(VI). Under conditions of pH (2), the addition amount of EFD&C (1 g/L), the adsorption time (760 min), and the temperature (45 °C), the maximum adsorption capacity reached 111.94 mg/g. The adsorption mechanism mainly attributed to chemical adsorption (redox), Cr(VI) reduced to Cr(III) by Fe(II) and Fe(0) (from Fe₃O₄ and Fe components in EFD) and surface functional groups of -OH, C = C, C-C and O-C = O (from biochar), and secondary attributed to physical adsorption, Cr(VI) and Cr(III) (from reduced Cr(VI)) adsorbed into the porous structure of EFD&C. This study provided a feasible solution for the preparation of adsorbents for adsorbing heavy metals from iron-containing metallurgical solid waste and biomass waste, which contributed to reducing the environmental pollution and lowering the cost of adsorbent preparation.

A metal-organic framework surrounded with conjugate acid-base pairs for the efficient capture of Cr(VI) via hydrogen bonding over a wide pH range

Given the large amount of toxic Cr(VI) wastewater from various industries, it is urgent to take effective treatment measures. Adsorption has been regarded as highly desirable for Cr(VI) removal, but the effectiveness of most adsorbents is significantly dependent on pH value, in which precipitous performance drop and even structural collapse generally occur in strong acidic/alkaline aqueous. Thus, maintaining high adsorption performance and structural integrity over a wide pH range is challenging. To efficiently remove Cr(VI), we designed and prepared of an acid-base resistant metal-organic framework (MOF) Zr-BDPO, by introducing weak acid-base groups (-NH-, -N= and -OH) onto the ligand. Zr-BDPO achieved a maximum adsorption capacity of 555.6 mg·g^-1 and retained skeletal structure at pH= 1-11. Interestingly, all these groups can generate conjugate acid-base pairs by means of H⁺ and OH^- in the external solution and then form buffer layer. The removal of Cr(VI) at a broad range of pH values primarily via hydrogen bonds between -NH- and -OH, and the oxoanion species of Cr(VI) is unusual. This strategy that insulating high concentrations of acids and bases and relying on hydrogen bonds to capture Cr(VI) oxoanions provides a new perspective for actual Cr(VI) wastewater treatment.

Preparation of Three-Dimensional MF/Ti3C2Tx/PmPD by Interfacial Polymerization for Efficient Hexavalent Chromium Removal

Heavy metal pollution is a serious threat to human health and the ecological environment, but adsorption technology based on nano adsorbents can effectively treat the crisis. However, due to the nanoscale effect, nano adsorbents have some crucial shortcomings, such as recycling difficulty and the loss of nanoparticles, which seriously limit their application. The feasible assembly of nano adsorbents is an accessible technology in urgent need of a breakthrough. In this study, three-dimensional (3D) adsorbent (MF/Ti3C2Tx/PmPD) with excellent performance and favorable recyclability was prepared by interfacial polymerization with melamine foam (MF) as the framework, two-dimensional (2D) titanium carbide (Ti3C2Tx) as the bridge and Poly (m-Phenylenediamine) (PmPD) as the active nano component. The morphology, structure, mechanical property of MF/Ti3C2Tx/PmPD and reference MF/PmPD were investigated through a scanning electron microscope (SEM), Fourier transformed infrared spectra (FT-IR), Raman scattering spectra and a pressure-stress test, respectively. Owning to the regulation of Ti3C2Tx on the morphology and structure of PmPD, MF/Ti3C2Tx/PmPD showed excellent adsorption capacity (352.15 mg/g) and favorable cycling performance. R-P and pseudo-second-order kinetics models could well describe the adsorption phenomenon, indicating that the adsorption process involved a composite process of single-layer and multi-layer adsorption and was dominated by chemical adsorption. In this research, the preparation mechanism of MF/Ti3C2Tx/PmPD and the adsorption process of Cr(VI) were systematically investigated, which provided a feasible approach for the feasible assembly and application of nano adsorbents in the environmental field.

Facile synthesis of sodium lignosulfonate/polyethyleneimine/sodium alginate beads with ultra-high adsorption capacity for Cr(VI) removal from water

Chromium (VI) is a widely occurring toxic heavy metal ion in industrial wastewater that seriously impacts the environment. In this study, we used environmentally friendly sodium lignosulfonate (SL), polyethyleneimine (PEI), and sodium alginate (SA) to synthesize SL/PEI/SA beads by employing a simple crosslinking method with to develop a novel absorbent with excellent adsorption capacity and practical application in wastewater treatment. We studied the adsorption performance of SL/PEI/SA through batch adsorption and continuous dynamic adsorption experiments. SL/PEI/SA has ultra-high adsorption capacity (2500 mg·g^-1) at 25 ℃, which is much higher than that of existing adsorbents. Humic acids and coexisting anions commonly found in wastewater have minimal effect on the adsorption performance of SL/PEI/SA. In the column system, 1 g SL/PEI/SA can treat 8.1 L secondary electroplating wastewater at a flow rate of 0.5 mLmin^-1, thereby enabling the concentration of Cr(VI) in secondary electroplating wastewater to meet the discharge standard (< 0.2 mg·L^-1). It is worth noting that the concentration of competitive ions in secondary electroplating wastewater is more than 500 times higher than that of Cr(VI). These results demonstrate that the novel SL/PEI/SA beads can be effectively applied in the removal of Cr(VI) in wastewater.

Development of Adsorptive Membranes for Selective Removal of Contaminants in Water

The presence of arsenic and ammonia in ground and surface waters has resulted in severe adverse effects to human health and the environment. Removal technologies for these contaminants include adsorption and membrane processes. However, materials with high selectivity and pressure stability still need to be developed. In this work, adsorbents and adsorptive membranes were prepared using nanostructured graphitic carbon nitride decorated with molecularly imprinted acrylate polymers templated for arsenate and ammonia. The developed adsorbent removed arsenate at a capacity and selectivity similar to commercial ion-exchange resins. Ammonia was removed at higher capacity than commercial ion exchange resins, but the adsorbent showed lower selectivity. Additionally, the prepared membranes removed more arsenate and ammonia than non-imprinted controls, even in competition with abundant ions in water. Further optimization is required to improve pressure stability and selectivity.

Adsorption of nitrate from water by quaternized chitosan wrinkled microspheres@MgFe-LDHs core-shell composite

In recent years, nitrate pollution in water became one of the global ecological problems. In this study, a new core-shell composite (GCS@CTA@MgFe-LDHs) was prepared by in-situ growth of MgFe-Cl--LDHs plates...

A Note about Crosslinking Density in Imprinting Polymerization

Imprinting polymerization is an exciting technique since it leads to specific binding sites, which are the basis of a variety of applications, such as sensors, detectors, and catalysts. The specific binding sites are created using templates and then fixing the structure of the binding site with crosslinking. The literature review of imprinting polymerizations shows that the crosslinking density governs the physical properties of the resulting molecularly imprinted polymer (MIP). It is also a factor governing the capacity and the selectivity of MIPs. Reviewing polymer science data and theory, the crosslinking density commonly used in MIP synthesis is unusually high. The data reviewed here suggest that more research is needed to determine the optimal crosslinking density for MIPs.