Organic arsenic adsorption onto a magnetic sorbent

Removal mechanism of Pb(ii) from soil by biochar-supported nanoscale zero-valent iron composite materials

As an adsorbent, biochar has a highly porous structure and strong adsorption capacity, and can effectively purify the environment. In response to the increasingly serious problem of heavy metal pollution in water, this study used nano zero valent iron and rice husk biochar to prepare a new type of magnetic sheet-like biochar loaded nano zero valent iron (BC-nZVI) composite material through rheological phase reaction, showing remarkable advantages such as low cost, easy preparation, and superior environmental remediation effect. The physical and chemical properties and structure of the material were extensively characterized using various methods such as HRTEM, XPS, FESEM, EDS, XRD, FTIR, and RAMAN. Concurrently, batch experiments were undertaken to assess the removal efficiency of Pb(ii) by BC-nZVI, with investigations into the influence of pH value, temperature, soil water ratio, and initial concentration of heavy metal ion solution on its removal efficiency. The results indicate that the removal of Pb(ii) by BC-nZVI reaches an equilibrium state after around 120 minutes. Under the conditions of pH 6, temperature 20 °C, soil water ratio 1 : 5, and BC-nZVI dosage of 1 g L-1, BC-nZVI can reduce the Pb(ii) content in wastewater with an initial concentration of 30 mg L-1 to trace levels, and the treatment time is about 120 minutes. The analysis of adsorption kinetics and isotherms indicates that the adsorption process of Pb(ii) by BC-nZVI adheres to the quasi-second-order kinetic model and Langmuir model, suggesting a chemical adsorption process. Thermodynamic findings reveal that the adsorption of Pb(ii) by BC-nZVI is spontaneous. Furthermore, BC-nZVI primarily accumulates Pb(ii) through adsorption co-precipitation. BC-nZVI serves as an eco-friendly, cost-effective, and highly efficient adsorbent, showing promising capabilities in mitigating Pb(ii) heavy metal pollution. Its recoverability and reusability facilitated by an external magnetic field make it advantageous for remediating and treating lead-contaminated sites.

Performance and mechanism of La-Fe metal-organic framework as a highly efficient adsorbent for fluoride removal from mine water

Water fluoride pollution has caused non-negligible harm to the environment and humans, and thus it is crucial to find a suitable treatment technology. In this study, La-Fe@PTA adsorbent was synthesized for the defluoridation of mine water. The results showed that the optimum conditions for defluoridation by La-Fe@PTA were pH close to 7.0, the initial F- concentration of 10 mg/L, the dosage of 0.5 g/L and the adsorption time of 240 min. Compared with SO42‒, Cl‒, NO3‒, Ca2+ and Mg2+, CO32‒ and HCO3‒ presented severer inhibition on fluoride uptake by La-Fe@PTA. The adsorption process fits well with the pseudo-second-order kinetic model and Freundlich model, and the maximum adsorption capacity of Langmuir model was 95 mg/g. Fixed-bed adsorption results indicated that fluoride in practical fluorinated mine water could be effectively removed from 3.6 mg/L to less than 1.5 mg/L within 130 bed volume (BV) by using 1.5 g La-Fe@PTA. Furthermore, the adsorbent still had good adsorption capacity after regeneration, which confirms the great application potential of La-Fe@PTA as a fluoride ion adsorbent. The mechanism analysis showed that La-Fe@PTA adsorption of fluorine ions is a physicochemical reaction driven by electrostatic attraction and ion exchange.

Iron-modified biochar improves plant physiology, soil nutritional status and mitigates Pb and Cd-hazard in wheat (Triticum aestivum L.)

Environmental quality and food safety is threatened by contamination of lead (Pb) and cadmium (Cd) heavy metals in agricultural soils. Therefore, it is necessary to develop effective techniques for remediation of such soils. In this study, we prepared iron-modified biochar (Fe-BC) which combines the unique characteristics of pristine biochar (BC) and iron. The current study investigated the effect of pristine and iron modified biochar (Fe-BC) on the nutritional values of soil and on the reduction of Pb and Cd toxicity in wheat plants (Triticum aestivum L.). The findings of present study exhibited that 2% Fe-BC treatments significantly increased the dry weights of roots, shoots, husk and grains by 148.2, 53.2, 64.2 and 148%, respectively compared to control plants. The 2% Fe-BC treatment also enhanced photosynthesis rate, transpiration rate, stomatal conductance, intercellular CO2, chlorophyll a and b contents, by 43.2, 88.4, 24.9, 32.5, 21.4, and 26.7%, respectively. Moreover, 2% Fe-BC treatment suppressed the oxidative stress in wheat plants by increasing superoxide dismutase (SOD) and catalase (CAT) by 62.4 and 69.2%, respectively. The results showed that 2% Fe-BC treatment significantly lowered Cd levels in wheat roots, shoots, husk, and grains by 23.7, 44.5, 33.2, and 76.3%. Whereas, Pb concentrations in wheat roots, shoots, husk, and grains decreased by 46.4, 49.4, 53.6, and 68.3%, respectively. Post-harvest soil analysis showed that soil treatment with 2% Fe-BC increased soil urease, CAT and acid phosphatase enzyme activities by 48.4, 74.4 and 117.3%, respectively. Similarly, 2% Fe-BC treatment significantly improved nutrients availability in the soil as the available N, P, K, and Fe contents increased by 22, 25, 7.3, and 13.3%, respectively. Fe-BC is a viable solution for the remediation of hazardous Cd and Pb contaminated soils, and improvement of soil fertility status.

Super-efficient removal and adsorption mechanism of anionic dyes from water by magnetic amino acid-functionalized diatomite/yttrium alginate hybrid beads as an eco-friendly composite

The development of eco-friendly, large-capacity and easy-to-separate adsorbent materials has always been the focus and difficulty of adsorption technology in wastewater treatment applications based on the characteristics of dye wastewater. Therefore, in this study, a green magnetic glycine(Gly)-functionalized diatomite(Dia)/yttrium alginate (Y-SA) hybrid bead composite (Dia-Gly-Y-SA@Fe₃O₄) was synthesized by the droplet polymerization, and characterized by various modern analytical techniques. The adsorption performance and adsorption mechanism of the composite were evaluated and elucidated by the removal of anionic dyes direct Blue 106 (DB 106), Congo red (CR) and direct red 13 (DR 13) from water. The results show that the composite is a macroparticle gelpolymer with an average particle size of about 1.5 mm, flower-like fold surface structure, abundant porosity and sensitive magnetic response, and displays ultrastrong adsorption ability for three dyes. The adsorption equilibrium of each dye can be reached quickly within 30 min, and the removal efficiency is more than 95% at pH 2.0 and decreases slightly with pH up to 9.0. The adsorption processes could be explained by the Pseudo-second-order rate equation well. All isotherm data fitted the Langmuir model well, and the maximum adsorption capacities were 1635, 2359 and 1165 mg/g for DB 106, CR and DR 13 at 298 K, respectively. The ultrastrong adsorption performance was due to the multisite interaction of physicochemical action and various hydrogen bonds between hybrid beads and dye anions. As a cost-effective magnetic macroparticle adsorbent prepared by natural ingredients, Dia-Gly-Y-SA@Fe₃O₄ composite exhibits much more stronger adsorption efficiency, better collectability and no secondary pollution than powder Dia, and would have a good application prospect for the purification of anionic dye wastewater with a wide pH range.

Adsorption of Copper and Arsenic from Water Using a Semi-Interpenetrating Polymer Network Based on Alginate and Chitosan

New biobased hydrogels were prepared via a semi-interpenetrating polymer network (semi-IPN) using polyacrylamide/chitosan (PAAM/chitosan) hydrogel for the adsorption of As(V) or poly acrylic acid/alginate (PAA/alginate) hydrogel for the adsorption of Cu(II). Both systems were crosslinked using N,N'-methylenebisacrylamide as the crosslinker and ammonium persulfate as the initiating agent. The hydrogels were characterized by SEM, Z-potential, and FTIR. Their performance was studied under different variables, such as the biopolymer effect, adsorbent dose, pH, contact time, and concentration of metal ions. The characterization of hydrogels revealed the morphology of the material, with and without biopolymers. In both cases, the added biopolymer provided porosity and cavities' formation, which improved the removal capacity. The Z-potential informed the surface charge of hydrogels, and the addition of biopolymers modified it, which explains the further metal removal ability. The FTIR spectra showed the functional groups of the hydrogels, confirming its chemical structure. In addition, the adsorption results showed that PAAM/chitosan can efficiently remove arsenic, reaching a capacity of 17.8 mg/g at pH 5.0, and it can also be regenerated by HNO₃ for six cycles. On the other hand, copper-ion absorption was studied on PAA/alginate, which can remove with an adsorption capacity of 63.59 mg/g at pH 4.0, and the results indicate that it can also be regenerated by HNO₃ for five cycles.

The Study of Optimal Adsorption Conditions of Phosphate on Fe-Modified Biochar by Response Surface Methodology

A batch of Fe-modified biochars MS (for soybean straw), MR (for rape straw), and MP (for peanut shell) were prepared by impregnating biochars pyrolyzed from three different raw biomass materials, i.e., peanut shell, soybean straw, and rape straw, with FeCl₃ solution in different Fe/C impregnation ratios (0, 0.112, 0.224, 0.448, 0.560, 0.672, and 0.896) in this research. Their characteristics (pH, porosities, surface morphologies, crystal structures, and interfacial chemical behaviors) and phosphate adsorption capacities and mechanisms were evaluated. The optimization of their phosphate removal efficiency (Y%) was analyzed using the response surface method. Our results indicated that MR, MP, and MS showed their best phosphate adsorption capacity at Fe/C ratios of 0.672, 0.672, and 0.560, respectively. Rapid phosphate removal was observed within the first few minutes and the equilibrium was attained by 12 h in all treatment. The optimal conditions for phosphorus removal were pH = 7.0, initial phosphate concentration = 132.64 mg L^-1, and ambient temperature = 25 °C, where the Y% values were 97.76, 90.23, and 86.23% of MS, MP, and MR, respectively. Among the three biochars, the maximum phosphate removal efficiency determined was 97.80%. The phosphate adsorption process of three modified biochars followed a pseudo-second-order adsorption kinetic model, indicating monolayer adsorption based on electrostatic adsorption or ion exchange. Thus, this study clarified the mechanism of phosphate adsorption by three Fe-modified biochar composites, which present as low-cost soil conditioners for rapid and sustainable phosphate removal.

Efficient removal of total arsenic (As3+/5+) from contaminated water by novel strategies mediated iron and plant extract activated waste flowers of marigold

In this investigation, marigold flower-waste was activated with iron salts (MG-Fe), subsequently marigold plant extract (MG-Fe-Ex) for the adsorptive elimination of As³⁺ and As⁵⁺ from contaminated water. The governing factor such as medium pH, temperature, pollutant concentration, reaction time, adsorbent dose were considered for the study. The complete elimination of As^3+/5+ was recorded with MG-Fe-Ex at pH 8.0, 90 min, 30 °C, dose 4 g/L, 20 mg/L of As^3+/5+ and shaking rate 120 rpm, while under the identical experimental condition, MG-Fe exhibited 98.4% and 73.3% removal for As⁵⁺ and As³⁺, respectively. The MG-Fe-Ex contains iron oxides (Fe₂O₃ and Fe₃O₄) as a result of iron ions reaction with plant bioactive molecules as evident from x-ray diffraction analysis (XRD), energy dispersive x-ray spectroscopic (EDS) and Fourier transform infrared (FTIR) spectroscopic study. The adsorption data of As^3+/5+ on MG-Fe and MG-Fe-Ex was best fitted by pseudo-first order kinetic and freundlich isotherm except As⁵⁺ adsorption on MG-Fe-Ex that can be described by langmuir isotherm model. The prevailing mechanism in adsorption of As^3+/5+ on both adsorbent might be hydrogen bonding, electrostatic attraction and complexation. From the above, it is confirmed that MG-Fe-Ex adsorbent has high potential and can be used for the adsorptive elimination of As^3+/5+ from contaminated water in sustainable and environmentally friendly way.

Enhanced Ciprofloxacin Removal from Aqueous Solution Using a Chemically Modified Biochar Derived from Bamboo Sawdust: Adsorption Process Optimization with Response Surface Methodology

Contamination of water by ciprofloxacin has become a significant concern due to its adverse health effects and growing evidence of antimicrobial-resistant gene evolution. To this end, a chemically modified bamboo biochar was prepared from bamboo sawdust to effectively remove ciprofloxacin (CIP) from an aqueous solution. Under similar adsorption conditions, the modified bamboo biochar (MBC) has an excellent CIP removal efficiency (96%) compared to unmodified bamboo biochar (UBC) efficiency (45%). Thus, MBC was used in batch adsorption experiments, and the process was optimized with the central composite design (CCD) framework of response surface methodology (RSM). Sorption process parameters such as initial CIP concentration, pH, adsorbent dose, and contact time were studied and found to have a significant effect on CIP removal. The optimal CIP removal (96%) was obtained at MBC dose (0.5 g L-1), CIP initial concentration (20 mg L-1), pH (7.5), and contact time (46 min). The adsorption kinetic data were well described by the pseudo-second-order model ( $R^2=0.999$ ), and both Langmuir ( $R^2=0.994$ ) and Freundlich ( $R^2=0.972$ ) models gave the best fit in CIP adsorption isotherm analysis. The maximum monolayer adsorption capacity of the MBC was 78.43 mg g-1 based on the Langmuir isotherm model. These results suggest that CIP adsorption was mainly controlled by chemisorption. Moreover, the CIP adsorption process was endothermic and spontaneous. Overall, MBC is a low-cost, efficient, and recyclable adsorbent for eliminating emerging contaminants such as ciprofloxacin from an aqueous solution.

Recovery of phosphate from aqueous solution by dewatered dry sludge biochar and its feasibility in fertilizer use

Discharging of dewatered dry sludge into the environment can trigger irreversible environmental damage. The treatment of dewatered dry sludge into a valuable biochar product is essential for the sustainability of resources and environmental safety. In this study, we examined the removal of phosphate from water bodies by adsorption of biochar that had been prepared from dewatered dry sludge with different pyrolysis temperatures. We showed that being a composite material rich in carbon, CaO and MgO were produced in the biochar preparation when the pyrolysis temperature was increased to 700 °C. The phosphate adsorption of biochar has strengthened with the increase of pH. The phosphate adsorption of composite was fitted with the pseudo-second-order kinetic model, while the Langmuir adsorption isotherm model yielded a maximum phosphate adsorption of 51.79 mg/g. The phosphate adsorption by sludge biochar was mainly affected by the deposition of phosphate crystals and electrostatic attraction. Overall, biochar prepared from dewatered dry sludge demonstrated acceptable phosphate removal performance and phosphate-loaded biochar had a slow release of phosphorus, therefore, can be used as a phosphate fertilizer to promote plant growth.

Adsorption of organic and inorganic arsenic from aqueous solution: Optimization, characterization and performance of Fe-Mn-Zr ternary magnetic sorbent

Arsenic is a highly toxic pollutant and exists in inorganic and organic forms in groundwater and industrial wastewater. It is of great importance to reduce the arsenic content to lower levels in the water (e.g., <10 ppb for drinking) in order to minimize risk to humans. In this study, a Fe-Mn-Zr ternary magnetic sorbent was fabricated via precipitation for removal of inorganic and organic arsenate. The synthesis of sorbent was optimized by Taguchi method, which leads to an adsorbent with higher adsorption capacity. The adsorption of As(V) was pH dependent; the optimal removal was achieved at pH 2 and 5 for inorganic and organic As(V), respectively. Contact time of 25 h was sufficient for complete adsorption of both inorganic and organic As(V). The adsorption isotherm study revealed that the adsorbent performed better in sequestration of inorganic As(V) than that of organic As(V); both adsorption followed the Langmuir isotherm with maximum adsorption capacities of 81.3 and 16.98 mg g^-1 for inorganic and organic As(V), respectively. The existence of anions in the water had more profound effect on the adsorption of organic As(V) than the inorganic As(V). The co-existing silicate and phosphate ions caused significantly negative impacts on the adsorption of both As(V). Furthermore, the existence of humic acid caused the deterioration of inorganic As(V) removal but showed insignificant impact on the organic As(V) adsorption. The mechanism study demonstrated that ion exchange and complexation played key roles in arsenic removal. This study provides a promising magnetic adsorptive material for simultaneous removal of inorganic and organic As(V).

Immobilization and transformation of co-existing arsenic and antimony in highly contaminated sediment by nano zero-valent iron

Arsenic (As) and antimony (Sb) are usually coexistent in mine wastes and pose a great threat to human health. The As immobilization by nano zero-valent iron (nZVI) is promising, however, the stabilization for co-occurring As and Sb is not known. Herein, the immobilization and transformation of As and Sb in nZVI-treated sediments were evaluated using complementary leaching experiments and characterization techniques. Raw sediment samples from a gold-antimony deposit revealed the co-existence of ultrahigh As and Sb at 50.3 and 14.9 g/kg, respectively. Leaching results show that As was more efficiently stabilized by nZVI than Sb, which was primarily due to the soluble fraction that was readily absorbed by nZVI of As was higher. As the nZVI treatment proceeds, the oxidation and reduction of As and Sb occur simultaneously as evidenced by XPS analysis. The primary oxidant, hydroxyl radicals, was detected by EPR studies, proving the occurrence of nZVI induced Fenton reaction. This study sheds light on differences in the interaction and immobilization of nZVI with Sb and As in co-contaminated sediments.

Self-enhanced and efficient removal of As(III) from water using Fe-Cu-Mn composite oxide under visible-light irradiation: Synergistic oxidation and mechanisms

Here, we prepared a novel nanostructured Fe-Cu-Mn composite oxide (FCMO_x) adsorbent using an ultrasonic coprecipitation method. The maximum adsorption capacity of As(III) and As(V) reached 158.5 and 115.2 mg/g under neutral conditions, respectively. The effects of several environmental factors (coexisting ions, solution pH, etc.) on the removal of inorganic arsenic using FCMO_x were studied through batch experiments. The results showed that except for PO₄^3- and high initial pH, it was not significantly affected by ionic strength and other existing anions, implying a higher selectivity and adaptability. Combined with EPR, FTIR, and XPS analysis, we concluded that the Cu component and the reactive oxygen species (ROS) it generates played a decisive role in maintaining the stability of the redox cycle between Mn(IV)/Mn(III)/Mn(II) and enhancing the oxidation efficiency of As(III). Meanwhile, the adsorption mechanism of As(V) was mainly through the replacement of the FCMO_x surface -OH to form stable inner-sphere arsenic complexes, while the removal mechanism of As(III) may involve the process of synergistic oxidation and chemisorption coupling. Additionally, the effective removal of As from the simulated As-contaminated water and its satisfactory reuse performance make FCMO_x adsorbents favorable candidates for the removal of As-contaminated water in the future.

In situ loading MnO2 onto 3D Aramid nanofiber aerogel as High-Performance lead adsorbent

Fabricating a high-performance adsorbent as a desirable candidate for removing Pb²⁺ from aqueous water remains a challenge. Aramid nanofibers (ANFs) are promising building blocks that have realized multifunctional applications due to their intrinsic mechanical and chemical stability. Herein, an in situ loading strategy for preparing nanofiber composite aerogel was proposed by assembling ANFs into a 3D aerogel and applying it as host media for the in situ polymerization of pyrrole followed by facile redox reaction between the polypyrrole (PPy) and MnO₄^-1 to load manganese dioxide (MnO₂). The idea was to fully exploit the structural advantages of ultra-low bulk density, large specific surface area, and high porosity of ANFs, and the possible chemical adsorption characteristics of MnO₂ on the basis of ion exchange reaction. The adsorption capacity of 3D ANF/MnO₂ composite aerogel was as large as 554.36 mg/g for Pb²⁺. The adsorption mechanism based on an exchange reaction between Pb²⁺ and protons on the surface of MnO₂ was also investigated. The desorption results showed that the adsorption performance could remain up to 90% after five times of usage. In conclusion, this research provides promising insights into the preparation of high-performance lead adsorbent for water treatment.

Fe3O4-NPs/orange peel composite as magnetic heterogeneous Fenton-like catalyst towards high-efficiency degradation of methyl orange

Magnetite nanoparticles (Fe₃O₄-NPs)/orange peel (MOP) composite was prepared via one-step in-situ co-precipitation method as magnetic heterogeneous Fenton-like catalyst. The properties of MOP were characterized by scanning electron microscopy, transmission electron microscopes, Brunauer-Emmett-Teller, X-ray diffraction, Fourier-transform infrared, thermogravimetric analysis and X-ray photoelectron spectroscopy technologies. Its Fenton-like catalytic responses towards removal of methyl orange (MO) were investigated, in which the effects of initial dye concentration, pH, temperature and hydrogen peroxide dosage were studied. The MO degradation ratio up to 98.0% was obtained within 20 min in optimized conditions. The catalyst showed excellent catalytic stability exhibiting nearly 90% degradation ratio in the 10th cycle within 20 min, whereas pure Fe₃O₄-NPs showed only 62.5% in this stage. Due to the stabilization of complexing orange peel hydroxyl to iron oxide in the composite and its magnetic separation property, MOP composite exhibits excellent Fenton-like catalytic performance, which offers great prospects for low-cost and high-efficiency organic dye wastewater treatment.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Fabrication of MOF-derivated CuOx-C electrode for electrochemical degradation of ceftazidime from aqueous solution

Antibiotic contamination has already been one of hazards to aquatic environment due to the abuse of antibiotics. Metal-organic frameworks (MOFs) are known as a kind of promising porous material for solving the environmental deterioration. In this article, the physicochemical and electrochemical properties of a series of porous copper oxide carbon materials (CuOx-C) synthesized by carbonizing Cu-BTC were compared. Due to the suitable carbonization temperature, CuOx-C-550 N, whose geometric structure was similar to Cu-BTC, possessed a multiscale pore structure containing many mesopores and partial macropores in accordance with the pore size distribution curves. More copper/copper oxides were introduced toimproving the electrochemical ability, evidence by XRD, XPS, CV and EIS characterization. Moreover, the degradation of ceftazidime (CAZ) through anodic oxidation was discussed. In AO/CuOx-C-550 N system, the effects of current, solution pH, initial CAZ concentration and Na₂SO₄ concentration were analyzed. CAZ removal rate reached 100% within 20 min under the optimal condition and a good electrocatalytic ability with 90% CAZ removal after 20 runs indicated a good electrochemical stability of CuOx-C-550 N. Furthermore, the degradation mechanism and pathway of CAZ were proposed. The Cu(II)/Cu(I) oxidation-reduction couples on the anodic surface contribute to the efficiently selective degradation of cephalosporins for CuOx-C-550 N. Overall, this study shows a good method to design and prepare a new MOF derivative for the remediation of aquatic contamination.

Characterization and application of magnetic biochar for the removal of phosphorus from water

Activated biochars were prepared from residues of medium density fiberboard (MDF) produced by the furniture industry. Biomass residue was pre-treated with FeCl3 in two different FeCl3:biomass ratios (0.5:1 and 1:1, w/w) aiming to produce a matrix embedded with iron oxide. The pyrolysis process produced maghemite on the biochar surface and its magnetic properties were confirmed by its attraction to a hand magnet and its magnetic susceptibility. Samples were also characterized using scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS), surface area by BET-N2, Fourier transform infrared (FTIR), X-ray diffraction, magnetic susceptibility, and cation exchange capacity (CEC). Magnetic biochar exhibited up to twelve-fold higher surface area than the non-magnetic biochar, which varies according the maghemite particles content. Iron oxide on biochar surface also contributed for increasing CEC around ten-fold compared to non-magnetic biochars. Phosphorus adsorption isotherms showed that these magnetic biochars have high capacity to sorb oxyanions like phosphate, especially at lower pH. Thus, these magnetic biochars could be used to clean water bodies contaminated with oxyanions in acidic conditions.

Goethite-modified biochar restricts the mobility and transfer of cadmium in soil-rice system

Cadmium (Cd) contamination of paddy soils has raised serious concerns for food safety and security. Remediation and management of Cd contaminated soil with biochar (BC) and modified biochar is a cost-effective method and has gained due attention in recent years. Goethite-modified biochar (GB) can combine the beneficial effects of BC and iron (Fe) for remediation of Cd contaminated soil. We probed the impact of different BC and GB amendments on Cd mobility and transfer in the soil-rice system. Both BC and GB effectively reduced Cd mobility and availability in the rhizosphere and improved the key growth attributes of rice. Although BC supply to rice plants enhanced their performance in contaminated soil but application of 1.5% GB to the soil resulted in prominent improvements in physiological and biochemical attributes of rice plants grown in Cd contaminated soil. Sequential extraction results depicted that BC and GB differentially enhanced the conversion of exchangeable Cd fractions to non-exchangeable Cd fractions thus restricted the Cd mobility and transfer in soil. Furthermore, supplementing the soil with 1.5% GB incremented the formation of iron plaque (Fe plaque) and boosted the Cd sequestration by Fe plaque. Increase in shoot and root biomass of rice plants after GB treatments positively correlates with incremented chlorophyll contents and gas exchange attributes. Additionally, the oxidative stress damage in rice plants was comparatively reduced under GB application. These findings demonstrate that amending the soil with 1.5% GB can be a potential remediation method to minimize Cd accumulation in paddy rice and thereby can protect human beings from Cd exposure.

Synthesis of Fe/Mg-Biochar Nanocomposites for Phosphate Removal

Magnetic biochar derived from agricultural biomass has been recognized as a cost-effective biochar sorbent for phosphate removal. This study evaluated the use of novel Fe/Mg-biochar nanocomposites (WBC1x), prepared by impregnating ground walnut shell in a solution with a different molar ratio of Fe2+ to Mg2+, then pyrolyzing slowly, at a temperature of 600 °C, to remove phosphate. The results showed that MgO and Fe3O4 were loaded onto the biochar successfully through the impregnation-pyrolysis method and the composites were able to be separated easily by magnetic field. Meanwhile, a higher surface area and point of zero charge on WBC1x were observed compared to the non-magnetic biochar (WBC). Moreover, the isothermal adsorption and kinetics data further suggested the that phosphate adsorption onto WBC1x resulted from chemisorption. Additionally, the maximum phosphate adsorption capacity of WBC1x was 6.9 mg.g-1, obtained though the Langmuir-Freundlich model, which was threefold higher than WBC, where MgO addition could enhance the adsorption capacity of WBC1x markedly by improving the surface charge.

Critical review of magnetic biosorbents: Their preparation, application, and regeneration for wastewater treatment

The utilisation of magnetic biosorbents (metal or metal nanoparticles impregnated onto biosorbents) has attracted increasing research attention due to their manipulable active sites, specific surface area, pore volume, pore size distribution, easy separation, and reusability that are suitable for remediation of heavy metal(loid)s and organic contaminants. The properties of magnetic biosorbents (MB) depend on the raw biomass, properties of metal nanoparticles, modification/synthesis methods, and process parameters which influence the performance of removal efficiency of organic and inorganic contaminants. There is a lack of information regarding the development of tailored materials for particular contaminants and the influence of specific characteristics. This review focuses on the synthesis/modification methods, application, and recycling of magnetic biosorbents. In particular, the mechanisms and the effect of sorbents properties on the adsorption capacity. Ion exchanges, electrostatic interaction, precipitation, and complexation are the dominant sorption mechanisms for ionic contaminants whereas hydrophobic interaction, interparticle diffusion, partition, and hydrogen bonding are the dominant adsorption mechanisms for removal of organic contaminants by magnetic biosorbents. In generally, low pyrolysis temperatures are suitable for ionic contaminants separation, whereas high pyrolysis temperatures are suitable for organic contaminants removal. Additionally, magnetic properties of the biosorbents are positively correlated with the pyrolysis temperatures. Metal-based functional groups of MB can contribute to an ion exchange reaction which influences the adsorption capacity of ionic contaminants and catalytic degradation of non-persistent organic contaminants. Metal modified biosorbents can enhance adsorption capacity of anionic contaminants significantly as metal nanoparticles are not occupying positively charged active sites of the biosorbents. Magnetic biosorbents are promising adsorbents in comparison with other adsorbents including commercially available activated carbon, and thermally and chemically modified biochar in terms of their removal capacity, rapid and easy magnetic separation which allow multiple reuse to minimize remediation cost of organic and inorganic contaminants from wastewater.

Preparation of Activated Carbon Supported Bead String Structure Nano Zero Valent Iron in a Polyethylene Glycol-Aqueous Solution and Its Efficient Treatment of Cr(VI) Wastewater

Nanometer zero-valent iron (nZVI) has been widely used in the treatment of heavy metals such as hexavalent chromium (Cr(VI)). A novel composite of bead string-structured nZVI on modified activated carbon (nZVI–MAC) is prepared here, using polyethylene glycol as the stable dispersant rather than traditional ethanol during the loading process. The microstructure characterization shows that nZVI particles are loaded on MAC with a bead string structure in large quantity and stably due to the addition of hydroxyl functional groups on the surface by polyethylene glycol. Experiments on the treatment of Cr(VI) in wastewater show that the reaction process requires only 20 min to achieve equilibrium. The removal rate of Cr(VI) with a low concentration (80–100 mg/L) is over 99% and the maximum saturation removal capacity is up to 66 mg/g. The system converts Cr(VI) to trivalent chromium (Cr(III)) through an oxidation-reduction effect and forms an insoluble material with iron ions by coprecipitation, which is then adsorbed on the surface of the nZVI–MAC. The process conforms to the quasi-second order adsorption kinetics equation (mainly chemical adsorption process).

Chemical Solution Deposition of Ordered 2D Arrays of Room-Temperature Ferrimagnetic Cobalt Ferrite Nanodots

Over the past decades, the development of nano-scale electronic devices and high-density memory storage media has raised the demand for low-cost fabrication methods of two-dimensional (2D) arrays of magnetic nanostructures. Here, we present a chemical solution deposition methodology to produce 2D arrays of cobalt ferrite (CFO) nanodots on Si substrates. Using thin films of four different self-assembled block copolymers as templates, ordered arrays of nanodots with four different characteristic dimensions were fabricated. The dot sizes and their long-range arrangement were studied with scanning electron microscopy (SEM) and grazing incident small-angle X-ray scattering (GISAXS). The structural evolution during UV/ozone treatment and the following thermal annealing was investigated through monitoring the atomic arrangement with X-ray absorption fine structure spectroscopy (EXAFS) and checking the morphology at each preparation step. The preparation method presented here obtains array types that exhibit thicknesses less than 10 nm and blocking temperatures above room temperature (e.g., 312 K for 20 nm diameter dots). Control over the average dot size allows observing an increase of the blocking temperature with increasing dot diameter. The nanodots present promising properties for room temperature data storage, especially if a better control over their size distribution will be achieved in the future.

Self-enhanced and efficient removal of arsenic from waste acid using magnetite as an in situ iron donator

High arsenic-containing waste acid from the heavy nonferrous metallurgical sector (Cu, Pb, Zn, Ni, Sn, etc.), one of the most dangerous arsenic hazardous wastes with extremely high arsenic concentrations, has presented enormous challenges to the environment and caused severe environmental pollution over the past few decades due to the lack of affordable and environmentally friendly disposal technologies. Here, we report a green process for the self-enhanced and efficient removal of arsenic from waste acid using magnetite as an in situ iron donator. Firstly, the room-temperature predissolution of magnetite in waste acid provides initial iron ions as a starting precipitator of arsenic, simultaneously providing a suitable pH range and an active surface that are ready for the nucleation and growth of scorodite. Afterwards, arsenic is precipitated in form the of scorodite, which is driven by a mutually improved cycle composed of arsenic precipitation and magnetite dissolution on the surface of magnetite particles. This cycle creates a low supersaturation of iron and constant pH in the waste acid, ensuring the continuous precipitation of arsenic as well-crystallized and environmentally stable scorodite by using magnetite as an in situ iron donator via the reaction of 2Fe₃O₄ + 6H₃AsO₄ + H₂O₂ = 6FeAsO₄ + 10H₂O. Under optimal conditions, including a 6-h room-temperature predissolution, a 12-h atmospheric reaction at 90 °C and a pH of 2.0 with a magnetite dosage at the Fe₃O₄/As molar ratio (the molar ratio of Fe₃O₄ in magnetite to As in waste acid) of 1.33, 99.90% of arsenic was successively removed from waste acid with an initial arsenic concentration of 10300 mg/L. In combination with the good adaptability of this process, the performed case study and prospective process show the successful removal of arsenic from waste acid as well as great potential for large-scale applications.

Preparation of Fe-Co based MOF-74 and its effective adsorption of arsenic from aqueous solution

To obtain a cost-effective adsorbent for the removal of arsenic in water, a novel nanostructured Fe-Co based metal organic framework (MOF-74) adsorbent was successfully prepared via a simple solvothermal method. The adsorption experiments showed that the optimal molar ratio of Fe/Co in the adsorbent was 2:1. The Fe₂Co₁ MOF-74 was characterized by various techniques and the results showed that the nanoparticle diameter ranged from 60 to 80 nm and the specific surface area was 147.82 m²/g. The isotherm and kinetic parameters of arsenic removal on Fe₂Co₁ MOF-74 were well-fitted by the Langmuir and pseudo-second-order models. The maximum adsorption capacities toward As(III) and As(V) were 266.52 and 292.29 mg/g, respectively. The presence of sulfate, carbonate and humic acid had no obvious effect on arsenic adsorption. However, coexisting phosphate significantly hindered the removal of arsenic, especially at high concentrations (10 mmol/L). Electrostatic interaction and hydroxyl and metal-oxygen groups played important roles in the adsorption of arsenic. Furthermore, the prepared adsorbent had stable adsorption ability after regeneration and when used in a real-water matrix. The excellent adsorption performance of Fe₂Co₁ MOF-74 material makes it a potentially promising adsorbent for the removal of arsenic.

Facile Synthesis of Graphene-Based Aerogels and Their Applications for Adsorption of Heavy Metal Ions

Graphene-based aerogels with a three-dimensional interconnected network were fabricated via the hydrothermal self-assembly and thermal-annealing process. The aerogels were characterized by means of scanning electron microscopy and atomic absorption spectroscopy. The graphene-based aerogels showed highly porous structure and adsorption capacity for heavy metal ions. Thus, they would be the promising materials for removal of heavy metal ions from water.

Enhanced phosphate sequestration by Fe(iii) modified biochar derived from coconut shell

In this work, a novel Fe-modified coconut shell biochar (Fe-CSB) was synthesized and utilized to remove phosphate from aqueous solution. Characterization results confirmed that the iron in the Fe(iii)-impregnated CSB existed mainly in the amorphous phase, as ferrihydrite and amorphous hydroxide, which substantially enhanced the phosphate adsorption. Batch experiments indicated that phosphate adsorption on the Fe-CSB was highly dependent on the pH, the humic acid, and temperature, while it was less affected by the nitrate. Phosphate adsorption by the CSB and Fe-CSB could be well described by the pseudo n-th order and Langmuir–Freundlich models. The fitting of the experimental data with the intra-particle diffusion model revealed that surface adsorption and inner-sphere diffusion were involved in the phosphate adsorption process, and that the latter was the rate-controlling step. Batch adsorption experiments and post-adsorption characterization results revealed that the phosphate adsorption by Fe-CSB was primarily governed by four mechanisms: ligand exchange, electrostatic attraction, chemical precipitation, and inner-sphere complexation. This work demonstrated that the modified Fe-CSB is an environmentally friendly and cost-effective bioretention medium and could open up new pathways for the removal of phosphorus from stormwater, as well as solve the problem of waste biomass pollution.

In this work, a novel Fe-modified coconut shell biochar (Fe-CSB) was synthesized and utilized to remove phosphate from aqueous solution.

Modification of biochar by Fe2O3 for the removal of pyridine and quinoline

The biochar produced by pyrolysis at 600°C from powdered mango bark impregnated with iron, namely Fe₂O₃/biochar, was recognized as a multifunctional material for environmental applications. This work aims at using the above material for the removal of pyridine and quinoline from aqueous solutions. The physico-chemical properties of synthesized biochars were studied systematically with several commonly used material characterization methods including scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FTIR). Structural and morphological analyses of the biochars showed that Fe₂O₃ particles were nanosized and distributed evenly on the surface of biochar, and thus exhibited great adsorption capacity for the removal of pyridine and quinoline. In addition, the results suggested that the main magnetic phase is maghemite (Fe₂O₃). The modified biochar showed high removal capacity (99%) for quinoline, but the removal capacity of pyridine was only 73%. The pH experiment in this passage indicated that both pyridine and quinoline reached a maximum adsorption amount at pH near 9.0. Adsorption kinetics suggested that the pseudo-second-order was well fitting the experiment data. The equilibrium study showed that the Langmuir model can adequately describe the adsorption process for pyridine and quinoline. The main mechanism of pyridine and quinoline removal was possibly electrostatic interactions and π-π interactions between the modified biochar and the adsorbate.

A Novel Multi-Charged Draw Solute That Removes Organic Arsenicals from Water in a Hybrid Membrane Process

The potential of forward osmosis for water treatment can only be maximized with suitable draw solutes. Here a three-dimensional, multicharge draw solute of decasodium phytate (Na₁₀-phytate) is designed and synthesized for removing organic arsenicals from water using a hybrid forward osmosis (FO) - membrane distillation (MD) process. Efficient water recovery is achieved using Na₁₀-phytate as a draw solute with a water flux of 20.0 LMH and negligible reverse solute diffusion when 1000 ppm organic arsenicals as the feed and operated under ambient conditions with FO mode. At 50 °C, the novel draw solute increases water flux by more than 30% with water fluxes higher than 26.0 LMH on the FO side, drastically enhancing water recovery efficiency. By combining the FO and MD processes into a single hybrid process, a 100% recovery of Na₁₀-phytate draw solute was achieved. Crucially, organic arsenicals or Na₁₀-phytate draw solutes are both rejected 100% and not detected in the permeate of the hybrid process. The complete rejection of both organic arsenicals and draw solutes using hybrid membrane processes is unprecedented; creating a new application for membrane separations.

Effectiveness and mechanisms of phosphate adsorption on iron-modified biochars derived from waste activated sludge

Excessive discharge of phosphate (P) into the surface water is the key factor to cause the eutrophication, so its removal has aroused much attention in recent years. In this study, different iron modification (chemical co-precipitation of Fe³⁺/Fe²⁺ or FeCl₃ impregnation) was used to improve the phosphate adsorption capacity of waste activated sludge (WAS)-based biochar. Comparative tests demonstrated that the FeCl₃-impregnated WAS-based biochar exhibited much superior phosphate adsorption capacity (111.0mg/g) in all as-prepared samples and performed well even under the interferences with pH and coexisting ions. X-ray diffraction (XRD) analyzes indicated that the iron in FeCl₃-impregnated WAS-based biochar existed mainly in amorphous phase, as hematite and amorphous hydroxides forms, which was of great benefit to the phosphate adsorption. Besides, ligand exchange plays important role in the adsorption of phosphate. The WAS-based biochar kept over 60% phosphate removal efficiency after five recycles.

Adsorption of organic arsenic acids from water over functionalized metal-organic frameworks

Organic arsenic acids (OAAs) are regarded as water pollutants because of their toxicity and considerable solubility in water. Adsorption of OAAs such as phenylarsonic acid (PAA) and p-arsanilic acid (ASA) from water was investigated over functionalized (with OH groups) metal-organic framework (MOF, MIL-101), as well as over pristine MIL-101 and commercial activated carbon. The highly porous MIL-101 bearing three hydroxyl groups (MIL-101(OH)₃) exhibited remarkable PAA and ASA adsorption capacities. Based on the effects of pH on PAA and ASA adsorption, hydrogen bonding was suggested as a plausible mechanism of OAA adsorption. Importantly, OAAs and MIL-101(OH)₃ can be viewed as hydrogen-bond acceptors and donors, respectively. Moreover, MIL-101(OH)₃ could be regenerated by acidic ethanol treatment, being a promising adsorbent for the removal of PAA and ASA from water.

Simultaneous enhancement of sludge dewaterability and removal of sludge-borne heavy metals through a novel oxidative leaching induced by nano-CaO2

The production of sewage sludge with the presence of various contaminants has been a serious issue for the operation of wastewater treatment plants on both the economical and environmental sides. To minimize the sludge volume to be handled and limit the potential environmental risk, this study developed a novel oxidative leaching process for enhanced sewage sludge dewatering and simultaneous removal of heavy metals based on nano-CaO₂. Response surface methodology determined the following optimal conditioning parameters in terms of capillary suction time reduction: 0.0906 g/g dry solid (DS) nano-CaO₂, 0.9969 mmol/g DS Fe²⁺, and pH of 5.59. The speciation partitioning analysis of the heavy metals pre and post nano-CaO₂ peroxidation indicated that the content of organically bound metals decreased and the percentage of soluble fraction increased substantially, which was beneficial for the removal of heavy metals through the dewatering unit. Nano-CaO₂ peroxidation could also induce the transformation of extracellular polymeric substances (EPS) from the tightly bound layers to the loosely bound layers of sewage sludge flocs. Through the decline of the Ryan-Weber constant of fluorescence titration and the pseudo-first-order kinetic constant of complexation, it was verified that the binding capacity of EPS with metal ions could be damaged by nano-CaO₂ peroxidation, which was the primary mechanism behind the substantial reduction of organically bound metals. This study is believed to provide novel insights into the application of nanotechnology in terms of the simultaneous volume and toxicity reduction of sewage sludge. Graphical abstract.

Low-cost magnetic herbal biochar: characterization and application for antibiotic removal

Magnetic biochar (M-BC) was derived from herbal medicine waste, Astragalus membranaceus residue, and was used as an adsorbent for ciprofloxacin removal from aqueous solutions. The M-BC was characterized by Brunauer-Emmett-Teller surface area analyses, Fourier transform infrared spectrometry, X-ray diffraction analysis, hysteresis loops, scanning electron microscopy energy-dispersive spectrometry, and X-ray photoelectron spectroscopy. The BET surface area increased from 4.40 to 203.70 m²/g after pyrolysis/magnetic modification. Batch experiments were performed at different dosages, initial concentrations, contact times, and solution pHs. Adsorption performances were evaluated using Langmuir and Freundlich isotherm models, and the results indicated that the Langmuir model appropriately described the adsorption process. The kinetic data were better fitted by a pseudo-second-order kinetic model. The maximum ciprofloxacin removal was observed at pH 6 (adsorption capacity of 68.9 ± 3.23 mg/g). Studies demonstrated that magnetically modified biochar might be an attractive, cost-effective, and easily separated adsorbent for contaminated water. Graphical abstract.

Calcium Silicate Phases Explained by High-Temperature-Resistant Phosphate Probe Molecules

In this work, high-temperature-resistant phosphate molecules are applied to characterize ultrathin (100 nm) calcium silicate (C-S) phases. These C-S phases are synthesized on silicon wafers, and the interaction of phosphates with the C-S phases is studied by means of in situ transmission Fourier transform infrared (FTIR) spectroscopy. At room temperature, the chemistry of the system is dominated by the formation of calcium phosphates (C-P). In the case of temperature rising to 1000 °C, the C-S phases are regenerated. FTIR results are analyzed on the basis of first-principles calculations and further supported by complementary time-of-flight secondary ion mass spectrometry (ToF-SIMS) experiments. This study provides a detailed and self-consistent picture of the chemical and structural properties of interfaces such as the one between the atmosphere and ultrathin C-S phases (gas/C-S) and the one between them and silicon wafers (C-S/Si bulk). The material combination of ultrathin C-S phases grown on silicon wafers might in the future have great potential in selective chemistry, catalysis, and sensing technology as well as in semiconductor manufacturing.

Removal of Hg(II) from aqueous solution using sodium humate as heavy metal capturing agent

An environmental friendly and economic natural biopolymer-sodium humate (HA-Na) was used to capture Hg(II) from aqueous solutions, and the trapped Hg(II) (HA-Na-Hg) was then removed by aluminium coagulation. The best Hg(II) capturing performance (90.60%) was observed under the following conditions: initial pH of 7.0, coagulation pH of 6.0, HA-Na dosage of 5.0 g L^-1, Al₂(SO₄)₃.18H₂O dosage of 4.0 g L^-1, initial Hg(II) concentration of 50 mg L^-1 and capturing time of 30 min. The HA-Na compositions with the molecular weight beyond 70 kDa showed the most intense affinity toward Hg(II). The results showed that the reaction equilibrium was achieved within 10 min (pH 7.0), and could be well fitted by the pseudo-second-order kinetics model. The capturing process could be well described by the Langmuir isotherm model and the maximum capturing capacity of Hg(II) was high up to 9.80 mg g^-1 at 298 K (pH 7.0). The Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis showed that the redox reaction between Hg(II) and HA-Na and the coordination reaction of carboxyl and hydroxy groups of HA-Na with Hg(II) were responsible for Hg(II) removal. The successive regeneration experiment showed that the capturing efficiency of humates for Hg(II) was maintained at about 51% after five capture-regeneration recycles.

Efficient conversion of dimethylarsinate into arsenic and its simultaneous adsorption removal over FeCx/N-doped carbon fiber composite in an electro-Fenton process

In this study, a FeCx/N-doped carbon fiber composite (FeCx/NCNFs) was developed via an electrospinning method. According to the characterization results of XRD, TEM and XPS, FeCx (a mixture of Fe7C3 and Fe3C) was either embedded in or attached to the NCNFs. It was used for the first time as a catalyst for dimethylarsinate (DMA) degradation and as an absorbent for inorganic arsenic (As (V)), with degradation and adsorption occurring simultaneously, in an electro-Fenton process. The effects of catalyst dosage, initial DMA concentration, solution pH, and applied current on the treatment efficiency and the corresponding H2O2 generation were systematically investigated. The results showed that DMA could be efficiently oxidized into As(V). 96% of DMA was degraded after reaction time of 360 min and the residual As(V) concentration in solution was below the allowable limit of 0.01 mg/L under the optimum treatment conditions. Based on an ESR and radical scavenger experiment, OH was proven to be the sole reactive oxygen species involved in the degradation process of DMA. DMA was oxidized to MMA as the primary oxidation product, which was subsequently oxidized to inorganic arsenic, As (V). TOC was also efficiently removed at the same time. The DMA removal mechanism for simultaneous degradation of dimethylarsinate and adsorption of arsenic over FeCx/NCNFs in the electro-Fenton process was also proposed based on the experimental results.

Functionalized chitosan electrospun nanofiber for effective removal of trace arsenate from water

An environment-friendly iron functionalized chitosan elctrospun nanofiber (ICS-ENF) was synthesized for trace arsenate removal from water. The ICS-ENF was fabricated by electrospinning a mixture of chitosan, PEO and Fe(3+) followed by crosslinking with ammonia vapor. The physicochemical properties of ICS-ENF were characterized by FESEM, TEM-EDX and XRD. The ICS-ENF was found to be highly effective for As(V) adsorption at neutral pH. The As(V) adsorption occurred rapidly and achieved equilibrium within 100 min, which was well fitted by pseudo-second-order kinetics model. The As(V) adsorption decreased with increased ionic strength, suggesting an outer-sphere complexation of As(V) on ICS-ENF. Freundlich model well described the adsorption isotherm, and the maximum adsorption capacity was up to 11.2 mg/g at pH 7.2. Coexisting anions of chloride and sulfate showed negligible influence on As(V) removal, but phosphate and silicate significantly reduced As(V) adsorption by competing for adsorption sites. FTIR and XPS analysis demonstrated -NH, -OH and C-O were responsible for As(V) uptake. ICS-ENF was easily regenerated using 0.003 M NaOH, and the removal rate remained above 98% after ten successively adsorption-desorption recycles. This study extends the potential applicability of electrospun nanofibers for water purification and provides a promising approach for As(V) removal from water.

A novel Fe–La-doped hierarchical porous silica magnetic adsorbent for phosphate removal

A novel Fe–La modified magnetic hierarchical porous silica was synthesized by an impregnation method to adsorb phosphate in aquatic ecology.

Comparative study on arsenate removal mechanism of MgO and MgO/TiO2 composites: FTIR and XPS analysis

The arsenate removal mechanism using MgO and MgO/TiO₂ adsorbents was revealed by Fourier transform infrared and X-ray photoelectron spectroscopy.

Coagulation of methylated arsenic from drinking water: Influence of methyl substitution

Methylated arsenic can be found in virtually all earth surface environments. So far, however, little information has been collected regarding their removal by coagulation. In this study, the removal of monomethylarsenate (MMA) and dimethylarsenate (DMA) from drinking water by coagulation was investigated from the viewpoint of methyl substitution. Results indicated that FeCl3 was more efficient than AlCl3 and polyaluminum chloride (PACl) in methylated As removal. For the initial arsenic concentration of 200 μg/L, an FeCl3 dosage of 0.2 mmol Fe/L was sufficient to attain about 95% removal of MMA, while a dosage of 0.6 mmol Fe/L achieved about 57% removal of DMA. Arsenic removal efficiency was negatively correlated with the degree of methyl substitution. With the increase in methyl group number, the quantity of negatively charged arsenic species decreased and molecular size increased, leading to the decrease of methylated As removal by coagulation. Adsorption on preformed hydroxide flocs was the major mechanism during coagulation. Both FTIR and XPS results indicated that the As−O group of As might substitute the O−H group of Fe/Al hydroxide to form a Fe/Al−O−As complex. Furthermore, the use of traditional oxidants and coagulation aids exhibited limited help for improving coagulation removal of DMA.

Enhanced Adsorption of Trivalent Arsenic from Water by Functionalized Diatom Silica Shells

The potential of porous diatom silica shells as a naturally abundant low-cost sorbent for the removal of arsenic in aqueous solutions was investigated in a batch study. The objective of this work was to chemically modify the silica shells of a diatom Melosira sp. with bifunctional (thiol and amino) groups to effectively remove arsenic in its toxic As(III) form (arsenite) predominant in the aquatic environment. Sorption experiments with this novel sorbent were conducted under varying conditions of pH, time, dosage, and As(III) concentration. A maximum adsorption capacity of 10.99 mg g^-1 was achieved within 26 h for a solution containing 12 mg L^-1 As(III) at pH 4 and sorbent dosage of 2 g L^-1. The functionalized diatom silica shells had a surface morphological change which was accompanied by increased pore size at the expense of reduced specific surface area and total pore volume. As(III) adsorption was best fitted with the Langmuir-Freundlich model, and the adsorption kinetic data using pore surface diffusion model showed that both the external (film) and internal (intraparticle) diffusion can be rate-determining for As(III) adsorption. Fourier transform infrared spectroscopy (FTIR) indicated that the thiol and amino groups potentially responsible for As(III) adsorption were grafted on the surface of diatom silica shells. X-ray photoelectron spectroscopy (XPS) further verified that this unique sorbent proceeded via a chemisorption mechanism through the exchange between oxygen-containing groups of neutral As(III) and thiol groups, and through the surface complexation between As(III) and protonated nitrogen and hydroxyl groups. Results indicate that this functionalized bioadsorbent with a high As(III) adsorption capacity holds promise for the treatment of As(III) containing wastewater.