A critical review on arsenic removal from water using iron-based adsorbents

Intensive research efforts have been pursued to remove arsenic (As) contamination from water with an intention to provide potable water to millions of people living in different countries. Recent studies have revealed that iron-based adsorbents, which are non-toxic, low cost, and easily accessible in large quantities, offer promising results for arsenic removal from water. This review is focused on the removal of arsenic from water using iron-based materials such as iron-based nanoparticles, iron-based layered double hydroxides (LDHs), zero-valent iron (ZVI), iron-doped activated carbon, iron-doped polymer/biomass materials, iron-doped inorganic minerals, and iron-containing combined metal oxides. This review also discusses readily available low-cost adsorbents such as natural cellulose materials, bio-wastes, and soils enriched with iron. Details on mathematical models dealing with adsorption, including thermodynamics, kinetics, and mass transfer process, are also discussed. For elucidating the adsorption mechanisms of specific adsorption of arsenic on the iron-based adsorbent, X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) are frequently used. Overall, iron-based adsorbents offer significant potential towards developing adsorbents for arsenic removal from water.

The recent developments on iron-based adsorbents such as iron oxyhydroxides nanoparticles, zero-valent iron, bimetallic oxides, and iron oxyhydroxide-doped composite materials are fully discussed in this review.     

Eco-friendly synthesis of graphene–chitosan composite hydrogel as efficient adsorbent for Congo red

A simple approach was utilized to synthesize graphene/chitosan-based hydrogel using glutaraldehyde as crosslinking agent in room temperature. The composite aerogel was used for removal of cationic and anionic dyes from aqueous solution. It showed high adsorption capacity towards Congo red as an anionic dye. Adsorption experiments were performed based on various parameters, such as initial Congo red concentration, solution pH and contact time. The kinetics data were analyzed using four different models and the pseudo-second-order model best described the adsorption of Congo red aerogel. The Equilibrium adsorption isotherm data indicated that equilibrium data were fitted to the Langmuir isotherm. The maximum dye adsorption capacity calculated from the Langmuir isotherm equation was 384.62 mg g⁻¹. Moreover, the aerogel was stable and easily recovered, and adsorption capacity was about 100% of the initial saturation adsorption capacity after being used three times.

Graphene/chitosan-based hydrogel was synthesized using glutaraldehyde as crosslinking agent in room temperature and it used for removal of Congo red dye from aqueous solution.     

Adsorption of aniline from aqueous solution using graphene oxide-modified attapulgite composites

Adsorption is an efficient treatment method for aniline removal in water treatment. In this work, the composites of graphene oxide-modified attapulgite were prepared and used firstly to remove aniline from wastewater. The composites were characterized by Fourier transformed infrared, Brunauer–Emmett–Teller, scanning electron microscopy and X-ray diffraction analysis. The effects of initial concentration, time, temperature and pH value on adsorption of aniline on graphene oxide-modified attapulgite are investigated. pH and temperature are found to have a significant influence on the adsorption amount. The experimental results showed that graphene oxide-modified attapulgite possesses strong adsorption ability for aniline with hydrogen bond interaction. The saturated adsorption amount could reach up to 90 mg g⁻¹ at pH = 2–4. The Langmuir isotherm is found to describe well the equilibrium adsorption data. Finally, graphene oxide-modified attapulgite is also observed to possess excellent reusability.

Adsorption is an efficient treatment method for aniline removal in water treatment.     

Removal of aqueous Hg(ii) by thiol-functionalized nonporous silica microspheres prepared by one-step sol–gel method

It is well known that thiol-functionalized silica (SiO₂-SH) can be used as an effective adsorbent for the removal of Hg(ii) from water. Studies in this field have focused on porous silica gels and mesoporous silicas that have large surface area and pore volume, while nonporous silica particles are seldom reported. This work aims to investigate the Hg(ii) adsorption properties of nonporous SiO₂-SH microspheres prepared by a simple one-step sol–gel method. The effects of pH, initial concentration of Hg(ii) and temperature on the adsorption properties of the SiO₂-SH microspheres were studied via batch adsorption experiments. The maximum adsorption capacity for Hg(ii) at 293 K calculated from the Langmuir equation was 377.36 mg g⁻¹. The adsorption kinetics and equilibrium data were well-fitted to the pseudo-second-order model and the Langmuir isotherm model, respectively.

The adsorption properties of nonporous SiO₂-SH microspheres prepared by a one-step sol–gel method for Hg(ii) in water were studied.     

Dicarboxylic acid cross-linked metal ion decorated bentonite clay and chitosan for fluoride removal studies

This study focused on the synthesis of a dicarboxylic acid (malic acid (A)), metal ion decorated bentonite clay (BC) modified with chitosan (CS) and the investigation of its defluoridation efficiency in fluoride contaminated groundwater. The synthesized adsorbent showed a fluoride removal capacity of 9.87 mg g⁻¹. Batch adsorption studies were conducted to establish the effect of various parameters such as contact time, pH, initial concentration, and competitor ions. The adsorption isotherms of Freundlich, Dubinin–Radushkevich, and Langmuir were studied and the Freundlich isotherm fitted the data well. Kinetic studies showed that the adsorption process follows pseudo second order kinetics. Thermodynamic studies revealed that the fluoride adsorption process is spontaneous and endothermic. Reuse and regeneration studies were executed for effective application of the nanocomposite. The synthesized adsorbent also has potential for real water treatment applications.

The adsorbent CeBC-A@CS nanocomposite has the maximum fluoride adsorption capacity.     

Comparative study of modified/non-modified aluminum and silica aerogels for anionic dye adsorption performance

Developing effective and low-cost adsorbents is of great significance for controlling water contamination. To eliminate anionic water contaminants, four modified/non-modified aluminum (Al_HMD, Al_OSSD) and silica (Si_HMD, Si_OSSD) aerogels have been successfully employed. The four as-prepared aerogels were applied as adsorbents for removal of an anionic dye (acid orange 7, AO) from aqueous solution. Compared to silica aerogels, aluminum aerogels showed efficient adsorption performance for anionic water contaminants. The AO maximum adsorption capacity of Al₂O₃ aerogel is twice as high as that of SiO₂ aerogel. The maximum adsorption capacity of aerogels was in the following order: Al_HMD > Al_OSSD > Si_HMD > Si_OSSD. Adsorption kinetics and isotherms of AO dye on the four as-prepared aerogels have also been studied. The kinetic data fitted well with the pseudo first-order kinetics model and the adsorption isotherm could be described by the Langmuir model. Adsorption rate of AO dye was mainly governed by film diffusion and intra-particle diffusion. Based on the adsorption mechanism, this work provides an idea for the design of superior adsorbents for anionic water contaminants.

Developing effective and low-cost adsorbents is of great significance for controlling water contamination.     

Preparation of activated carbon from Dipterocarpus alatus fruit and its application for methylene blue adsorption

Activated carbons were prepared from three parts of Dipterocarpus alatus fruit (wing, endocarp and pericarp), an abundant and renewable waste in Southeast Asia, by chemical activation using ZnCl₂, FeCl₃, H₃PO₄ and KOH and physical activation using CO₂ and steam. This study indicated that activated carbon prepared from Dipterocarpus alatus fruit could be employed as a promising adsorbent for the removal of methylene blue from aqueous solution. ZnCl₂ activation led to an activated carbon with a surface area of 843 m² g⁻¹ and was able to remove methylene blue from aqueous solution. Adsorption studies were performed and analysed using Langmuir and Freundlich isotherm equations. Adsorption data demonstrated an excellent fit with the Langmuir isotherm model, with the maximum adsorption capacity of 269.3 mg g⁻¹ at equilibrium. Pseudo-first order and pseudo-second order kinetic models were used in this study to describe the adsorption mechanism. The results show that methylene blue adsorption is pseudo-second order, indicating that liquid film diffusion, intra-particle diffusion and surface adsorption coexisted during methylene blue adsorption on the activated carbon. The activated carbon prepared from Dipterocarpus alatus fruit is a low cost and effective adsorbent with a fast rate for the removal of methylene blue from aqueous solutions when compared with a number of activated carbons studied in the literature.

Activated carbons were prepared from Dipterocarpus alatus fruit by chemical and physical activation and used for the removal of methylene blue from aqueous solution.     

Superb adsorption capacity of biochar derived from leather shavings for Congo red

Research on biochar for removal of dyes has been a hot topic because of its excellent eco-friendly and economical properties. In this study, leather shavings biochar (LSB) with high adsorption capacity was prepared and tested with Congo red as a model dye for adsorption. The research results show that the as-prepared biochar exhibits a porous structure, with a high specific surface area (2365 m² g⁻¹), and it would be beneficial for removing Congo red from effluents. More interestingly, adsorption capacity of LSB for Congo red was enhanced by chromium compounds on the surface of biochar through chelation and electrostatic interactions. Chelation occured between the chromium compounds and amino groups of Congo red. Adsorption data for Congo red on the biochar were successfully described by Langmuir isotherm and the pseudo-second order kinetics model. Langmuir maximum adsorption capacity of LSB at 30 °C reached 1916 mg g⁻¹, which is much higher than that of conventional activated carbon (AC). Recycling experiment shows that LSB has a potential market for removing Congo red.

Leather shavings biochar can achieve high adsorption performance for Congo red through physical and chemical interactions.     

Adsorptive removal of antibiotics from water using peanut shells from agricultural waste

The residues of sulfonamides in the environment have received widespread attention because of their potential hazards. In this study, the potential of peanut shells for antibiotic removal from aqueous solutions was investigated for four antibiotics (sulfamerazine, sulfamethazine, sulfathiazole, and sulfamethoxazole). The properties of the peanut shells were characterized using Brunauer–Emmett–Teller method, X-ray photoelectron spectroscopy, scanning electron microscopy and Fourier-transform infrared spectroscopy analyses, and the results of the analyses showed that the significant properties of peanut shells were mainly attributed to the antibiotics'' adsorption process. A batch adsorption experiment was conducted to study the effect of dosage, concentration, and water factors (Fe³⁺, Mn²⁺, and Ca²⁺) on antibiotic adsorption. Adsorption kinetics and isotherms were also studied. The kinetic data showed that a pseudo-second order kinetic model fitted the experimental data, the adsorption isotherm experimental data fitted the Henry linear adsorption model well, and methanol was found to be an effective eluent for desorption of the antibiotics. The results indicate that peanut shells are a promising material for the removal of antibiotics from contaminated water, when present at low initial concentrations.

The porous structure of the materials was determined using the surface area and pore size analyzer (JW-BK132F) by N₂ adsorption–desorption isotherms at 77 K.     

Adsorptive,kinetics and regeneration studies of fluoride removal from water using zirconium-based metal organic frameworks

Fluoride contamination has been recognised as one of the major problems worldwide, imposing a serious threat to human health and affecting the safety of drinking water. Adsorption is one of the widely considered appropriate technologies for water defluorination. The present study describes the preparation of a zirconium-based metal organic framework (MOF-801) adsorbent using a solvothermal method and its adsorption efficiency for removal of fluoride ions from water. The morphology of MOF-801 was characterized by PXRD, FESEM and XPS, and the pore structure and surface area were calculated according to BET. It was found that the synthesized MOF-801 showed the distinguishable octahedral shape particle with a lattice spacing of 0.304 nm, indicative of (011) planes of ZrO₂. Adsorption studies were carried out to study the defluorination effectiveness by varying contact time (30–150 min), adsorbent dose (0.3–1.5 g L⁻¹), adsorbate concentration (5–25 mg L⁻¹), as well as kinetics and isotherms. The maximum removal efficiency for fluoride using MOF-801 at equilibrium was found to be 92.3%. Moreover, the adsorption kinetic studies indicate that the overall fluoride adsorption process was best described by pseudo-second-order kinetics. The adsorption data were well-fitted with the Langmuir isotherm model (R² = 0.9925) with maximum adsorption capacity of 19.42 mg g⁻¹. The synthesized MOF-801 had good reusability and was used in up to four cycles for fluoride removal attaining around 79% removal efficiency after the fourth cycle. All the results suggested that the synthesized MOF-801 has potential to be an excellent adsorbent for wastewater defluorination treatment.

A facile solvothermal method is used to prepare octahedral MOF-801 with a lattice spacing of 0.304 nm representative of ZrO₂ (011) planes for water defluorination.     

Prospective application of phosphorylated carbon nanofibers with a high adsorption capacity for the sequestration of uranium from ground water

In this study carbon nanofibers (CNF) were phosphorylated by using ortho-phosphoric acid and applied for adsorptive remediation of uranium from water. The phosphorylated carbon nanofibers (PCNF) showed 96% removal of uranium as compared to 79% by CNF. The adsorption data from batch adsorption studies fitted well with the Langmuir model and a maximum adsorption capacity of 512.8 mg g⁻¹ was obtained at pH 6.0 while the adsorption followed pseudo second order kinetics. A detailed characterisation of the adsorbent has been carried out using various analytical and spectroscopic tools. The application of the adsorbent to ground water samples exhibited promising results even in the presence of other interfering cations and anions which is imperative considering the toxic effects of uranium in ground water.

Adsorption of uranium at pH 6.0 using phosphorylated carbon nanofibers.     

Acetone fractionation: a simple and efficient method to improve the performance of lignin for dye pollutant removal

In this work, it was found that the adsorption capacity of lignin to cationic dye (methylene blue, MB) from aqueous solution could be significantly improved by simple acetone fractionation. The removal efficiency of MB by acetone insoluble kraft lignin (AIKL) was 10 times that of unfractionated kraft lignin (KL). And the maximum capacity of AIKL could reach up to 623.4 mg g⁻¹. And the high removal rate could be achieved even at low concentrations. The effects of ionic strength, temperature, adsorbent dosage were systematically investigated. Adsorption kinetics showed the adsorption behavior obeyed the pseudo-second-order kinetic model. The equilibrium data was more consistent with the Langmuir isotherm model. Thermodynamic analyses proved that the adsorption was a spontaneous and endothermic physisorption process. In addition, the reasons for the enhanced adsorption effect by fractionation were clarified based on characterization by FT-IR. The enhancement of π–π interaction between AIKL and MB caused by fractionation plays an important role in the adsorption process.

In this work, it was found that the adsorption capacity of lignin to cationic dye (methylene blue, MB) from aqueous solution could be significantly improved by simple acetone fractionation.     

Functionalised electrospun membranes (TETA-PVC) for the removal of lead(ii) from water

Driven by the need for delivering sustainable water purification solutions for the removal of heavy metals from water, electrospun PVC membranes were functionalised with triethylenetetramine (TETA) and were used to remove lead(ii) ions selectively from water. The membranes were characterised and their adsorption behavior towards the removal of lead from water was investigated. The incorporation of TETA on the membrane''s surface significantly improved the removal efficiency of lead(ii) up to 99.8% in 30 minutes and under ambient conditions, with the lowest concentration of 50 ppm. The adsorption mechanism was investigated and kinetic data showed a better correlation with the pseudo-second-order model. Similarly, the equilibrium data best fitted with the Langmuir adsorption isotherm model with a relatively high maximum adsorption capacity of 1250 mg g⁻¹ for lead(ii) ions, larger than recently reported adsorption capacities for similar membranes. The functionalised membrane also showed high selectivity to lead(ii) in a mixed solution containing lead(ii), mercury(ii), cadmium(ii), arsenic(iii), copper(ii), and zinc(ii). The functionalised membrane was regenerated, where desorption of lead(ii) was achieved, under mildly acidic conditions. The removal efficiency of the regenerated membrane after six cycles of adsorption/desorption was maintained at a high level of 98%. The proposed design offers a simple yet effective, sustainable, and environmentally friendly solution for water treatment.

Driven by the need for delivering sustainable water purification solutions for the removal of heavy metals from water, TETA functionalised electrospun PVC membranes were fabricated and used to remove lead(ii) ions selectively from water.     

Efficient removal of Cu(ii) from aqueous systems using enhanced quantum yield nitrogen-doped carbon nanodots

The valorization of cellulose-based waste is of prime significance to green chemistry. However, the full exploitation of these lignocellulosic compounds to produce highly luminescent nanoparticles under mild conditions has not yet been achieved. In this context, we convert low-quality waste into value-added nanomaterials for the removal of Cu(ii) from wastewater. Carboxymethylcellulose (CMC), which was derived from empty fruit bunches, was selected for its high polymerization index to produce luminescent nitrogen-doped carbon dots (N-CDs) with the assistance of polyethylene glycol (PEG) as a dopant. The optimum N-CD sample with the highest quantum yield (QY) was characterized using various analytical techniques and the results show that the N-CDs have great crystallinity, are enriched with active sites and exhibit a long-shelf life with an enhanced QY of up to 27%. The influence of Cu²⁺ concentration, adsorbent (N-CDs) dosage, pH and contact time were investigated for the optimal adsorption of Cu²⁺. The experiments showed the rapid adsorption of Cu²⁺ within 30 min with a removal efficiency of over 83% under optimal conditions. The equilibrium isotherm investigation revealed that the fitness of the Langmuir isotherm model and kinetic data could be well explained by the pseudo-second order model. Desorption experiments proved that N-CDs can be regenerated successfully over five adsorption–desorption cycles owing to the ability of ascorbic acid (AA) to reduce the adsorbed nanocomplex into Cu⁺. The rapid adsorption property using low-cost materials identifies N-CDs as a superior candidate for water remedy.

Low value waste resources have been converted into value-added luminescence carbon dots for copper adsorption from contaminated water.     

Optimum selective separation of Cu(ii) using 3D ordered macroporous chitosan films with different pore sizes

Although bio-materials are widely used for the adsorption of heavy metal ions, they have low specific surface area, slow adsorption rates, and poor selectivity. To overcome these limitations, in this study, we report a 3D-ordered macroporous ion-imprinted chitosan film (3DOM-IICF); the 3DOM-IICF coupled with a colloidal crystal template and ion imprinting (IIP) technology has been used to absorb copper ions (Cu(ii)) present in water. Moreover, polystyrene (PS) micro-spheres and copper templates were added to form a three-dimensional ordered macroporous structure and ion-imprinted sites, respectively. Finally, the film was formed by drying. Adsorption experiments showed the removal of Cu(ii) from the 3DOM-IICF in an aqueous solution. The 3DOM-IICF exhibited good adsorption performance under neutral conditions of pH = 7.0, and the adsorption efficiency was high. The maximum adsorption capacity of the 3DOM-IICF was 261.31 mg g⁻¹. The adsorption processes were more consistent with the pseudo-second-order kinetic model and the Langmuir isotherm. The 3DOM-IICF exhibits superior selective adsorption of Cu(ii). Moreover, the 3DOM-IICF could be regenerated multiple times, reused as an adsorbent and maintained high adsorption capacity. This kind of imprinted template method has important significance in the selective adsorption of pollutants in bio-materials and is worthy of in-depth research.

3DOM-IICF coupled with colloidal crystal template and ion imprinting technology (IIP) was used to absorb copper ions (Cu(ii)) in water. Added polystyrene (PS) micro-spheres to form a three-dimensional ordered macroporous structure.     

The preparation of organophosphorus ligand-modified SBA-15 for effective adsorption of Congo red and Reactive red 2

P,P-bis (2-oxooxazolidin-3-yl)-N-(3-(triethoxysilyl)propyl)phosphinic amide (APTES-BOP)-modified SBA-15 (SBA-15-BOP) was prepared by a post-synthesis grafting method for the removal of anionic azo dyes from aqueous solutions. The properties of the prepared adsorbent were characterized by PXRD, FT-IR, SEM, TEM, nitrogen sorption, and elemental analysis. Adsorption equilibrium and adsorption kinetic studies demonstrated that the experimental data fitted well with the Langmuir isotherm model and pseudo-second-order model. According to Langmuir fitting, SBA-15-BOP showed high adsorption capacity for CR and RR2 dyes, with the maximum adsorption capacities of 518.1 mg g⁻¹ and 253.8 mg g⁻¹, respectively. The thermodynamic study indicated that the adsorption processes of CR and RR2 dyes on SBA-15-BOP were spontaneous and exothermal. The prepared SBA-15-BOP can be a promising adsorbent for the removal of anionic dyes from aqueous solutions.

P,P-bis (2-oxooxazolidin-3-yl)-N-(3-(triethoxysilyl)propyl)phosphinic amide (APTES-BOP)-modified SBA-15 (SBA-15-BOP) was prepared by a post-synthesis grafting method for the removal of anionic azo dyes from aqueous solutions.     

Nanocomposite functionalized membranes based on silica nanoparticles cross-linked to electrospun nanofibrous support for arsenic(v) adsorption from contaminated underground water

Nanocomposite functionalized membranes were synthesized using surface functionalized mesoporous silica nanoparticles (MCM-NH₂ or MCM-PEI) cross-linked to a modified polyacrylonitrile (mPAN) nanofibrous substrate for the removal of 1 mg L⁻¹ of As(v); a concentration much higher than what has been reported for underground water in Argentina. Adsorption studies were carried out in batch mode at pH 8 with nanoparticles in colloidal form, as well as the nanoparticles supported on the modified PAN membranes (mPAN/MCM-NH₂ and mPAN/MCM-PEI). Results indicate a twenty-fold improvement in As(v) adsorption with supported nanoparticles (nanocomposite membranes) as opposed to their colloidal form. The adsorption efficiency could be further enhanced by modifying the nanocomposite membrane surface with Fe³⁺ (mPAN/MCM-NH₂-Fe³⁺ and mPAN/MCM-PEI-Fe³⁺) which resulted in more than 95% arsenic being removed within the first 15 minutes and a specific arsenic adsorption capacity of 4.61 mg g⁻¹ and 5.89 mg g⁻¹ for mPAN/MCM-NH₂-Fe³⁺ and mPAN/MCM-PEI-Fe³⁺ nanocomposite membranes, respectively. The adsorption characteristics were observed to follow a pseudo-first order behavior. The results suggest that the synthesized materials are excellent for quick and efficient reduction of As(v) concentrations below the WHO guidelines and show promise for future applications.

Development of nanocomposite functionalized membranes for the removal of As(v) from contaminated water.     

Enhanced adsorption behaviors of Co2+ on robust chitosan hydrogel microspheres derived from an alkali solution system: kinetics and isotherm analysis

Chitosan hydrogel microspheres derived from the LiOH/KOH/urea aqueous system demonstrate great characteristics of high mechanical strength, relative chemical inertness, renewability and 3-D fibrous network, making them promising functional supports. This work aims to investigate the tunable Co²⁺ adsorption behaviors on these robust chitosan microspheres in detail, providing the theoretical basis for optimizing the preparation procedure of chitosan microspheres supported Co₃O₄ catalysts in the future. The experimental results revealed that the fabricated original chitosan microspheres with more extended chain conformation could display enhanced adsorption capacity for Co²⁺ at determined concentration both in water and alcohol solutions, which is about 2–7 times higher than that of the conventional chitosan hydrogel microspheres prepared from the acetic acid solution. The kinetic experiments indicated that the adsorption process in water solution agreed with the pseudo-second-order kinetic equation mostly, while the chemical and physical adsorptions commonly contribute to the higher Co²⁺ adsorption on chitosan microspheres in alcohol solution. Moreover, in both cases, the film diffusion or chemical reaction is the rate limiting process in the initial adsorption stage, and the adsorption of Co²⁺ on chitosan microspheres can well fit to the Langmuir isotherm. Thermodynamic analysis demonstrated that such adsorption behaviors were dominated by an endothermic (ΔH° > 0) and spontaneous (ΔG° < 0) process.

The 3-D fibrous network endows the chitosan hydrogel microspheres fabricated from the alkaline solvent system with high mechanical strength and enhanced adsorption capacity of Co²⁺, making them as the ideal and stable catalyst supports.     

Insight into the synergistic effect on adsorption for Cr(vi) by a polypyrrole-based composite

Polypyrrole-based (PPy) composite are promising candidates for the treatment of water pollution. Adsorption selectivity as well as a large adsorption capacity are two key factors for treating wastewater containing multiple ions. The structure and morphology of the prepared composites were characterized by the FT-IR, XRD and SEM examinations. The results indicate that the Fe₃O₄ and PPy nanosphere coats attapulgite (ATP) closely and evenly. Herein, a novel Fe₃O₄ and ATP doped three-dimensional network structure PPy/Fe₃O₄/ATP composite was demonstrated as an excellent adsorbent to effectively remove Cr(vi). The as-synthesized PPy/Fe₃O₄/ATP composite is suitable for Cr(vi) adsorption in a wide pH range (pH 2–6). Up to a 96.44% removal rate was found with 400 mg L⁻¹ Cr(vi) aqueous solution in 30 min for 0.2 g PPy/Fe₃O₄/ATP adsorbent. Adsorption results showed that Cr(vi) removal efficiency by PPy/Fe₃O₄/ATP decreased with an increase in pH. The removal rate of Cr(vi) had already reached 93.63% in 15 min contact time. Co-existing ions studies exhibit inorganic oxyacid anion and transition metal cation showed negative effects on Cr(vi) removal rate. A chemical rather than a physical adsorption occurred for these adsorbents as revealed by a pseudo-second-order kinetic study. The results of the adsorption isotherms showed that the adsorption process was similar to the Langmuir isotherm adsorption. Furthermore, the PPy/Fe₃O₄/ATP composite exhibited a high stability for Cr(vi) adsorption during recycling tests process. This work may provide some useful guidelines for designing adsorbents with selectivity toward specific heavy metal ions.

Polypyrrole-based (PPy) composite are promising candidates for the treatment of water pollution.     

Performance of a zeolite modified with N,N-dimethyl dehydroabietylamine oxide (DAAO) for adsorption of humic acid assessed in batch and fixed bed columns

Factors that affect adsorption of a synthetic humic acid (HA) on a zeolite modified with the surfactant N,N-dimethyl dehydroabietylamine oxide (DAAO) (SMZ) were investigated in batch and fixed bed column experiments. Adsorption increased with increasing HA concentrations and contact time, but decreased with increasing pH, temperature, and ionic strength. Adsorption of HA on SMZ was increased by the presence of the cations Ca²⁺ and Mg²⁺, whereas anions such as NO₃⁻, HCO₃⁻, SO₄²⁻, and PO₄³⁻ showed the opposite trend, and competed with HA adsorption. HA adsorption on SMZ was well-fitted by pseudo-second order kinetics, and described by the Langmuir isotherm model. The maximum adsorption capacity in batch experiments calculated from the Langmuir adsorption isotherm was about 126 mg g⁻¹. Thermodynamic calculations showed that HA adsorption on the zeolite with bilayer DAAO coverage was spontaneous and exothermic. Optimum desorption was obtained using 0.1 M NaOH with a recovery of 94%. The HA adsorption capacity of SMZ at the breakthrough point was greatly influenced by bed depth, and could be described by the Thomas model. Adsorption mechanisms are interpreted as involving mainly hydrogen bonding and electrostatic interactions.

Factors that affect adsorption of a synthetic humic acid (HA) on a zeolite modified with the surfactant N,N-dimethyl dehydroabietylamine oxide (DAAO) (SMZ) were investigated in batch and fixed bed column experiments.