Adsorption of copper-citrate complexes on chitosan: equilibrium modeling

Biosolids-based activated carbon for enhanced copper removal from citric-acid-rich aqueous media

In this study, we employed batch experiments to assess the effects of citric acid on the Cu(II) removal efficiencies of seven biosolids-based adsorbents. The adsorbents used were dried biosolids (BS), biosolids biochar (BSBC), biosolids-based activated carbon (SBAC), nitric-acid-modified BSBC (BSBC-HNO₃) and SBAC (SBAC-HNO₃), and amine-modified BSBC (BSBC-NH₂) and SBAC (SBAC-NH₂). However, with 100 mM citric acid in 1 mM Cu(II) solution, only SBAC showed an increase in Cu(II) removal efficiency (64.0-93.5%). Therefore, we used SBAC for further optimisation of the adsorption process. The kinetics data, optimally described by the pseudo-second-order model, indicated that bulk Cu(II) adsorption occurred within 10 min. The highest Cu(II) removal was at pH 3, with the estimated maximum Cu(II) adsorption capacity of SBAC increasing from 0.14 to 0.30 mmol/g, with 100 mM citric acid present. This result clearly indicated the positive effect of citric acid on Cu(II) adsorption. With citric acid present, the Freundlich model optimally fitted the adsorption isotherm data, suggesting adsorption of Cu(II) in multilayers. Further investigation of Cu(II) adsorption in a sequential setup proved that SBAC could lower the residual Cu(II) in the solution to below the discharge limit (0.05 mM) in 1 h. Overall, the production of activated carbon from BS has been proven an efficient Cu(II) adsorbent for Cu-citric-acid-rich aqueous media as a simulation of real wastewaters/leachates, as well as achieving waste-to-resources goals. This is the first study to identify an adsorbent (SBAC) with increased Cu(II) adsorption capacity in the presence of excess citric acid.

Design of High-Relaxivity Polyelectrolyte Nanocapsules Based on Citrate Complexes of Gadolinium(III) of Unusual Composition

Through nuclear magnetic relaxation and pH-metry, the details of the complexation of gadolinium(III) ions with citric acid (H₄L) in water and aqueous solutions of cationic polyelectrolytes are established. It is shown that the presence of poly(ethylene imine) (PEI) in solution affects magnetic relaxation behavior of gadolinium(III) complexes with citric acid (Cit) to a greater extent than polydiallyldimethylammonium chloride (PDDC). A large increase in relaxivity (up to 50 mM⁻¹s⁻¹) in the broad pH range (4–8) is revealed for the gadolinium(III)–citric acid–PEI system, which is particularly strong in the case of PEI with the molecular weight of 25 and 60 kDa. In weakly acidic medium (pH 3–7), the presence of PEI results in the formation of two tris-ligand associates [Gd(H₂L)₃]³⁻ and [Gd(H₂L)₂(HL)]⁴⁻, which do not exist in aqueous medium. In weakly alkaline medium (pH 7–10), formation of ternary complexes Gd(III)–Cit–PEI with the Gd(III)–to–Cit ratio of 1:2 is evidenced. Using transmission electron microscopy (TEM) and dynamic light scattering techniques (DLS), the formation of the particles with the size of 50–100 nm possessing narrow molecular-mass distribution (PDI 0.08) is determined in the solution containing associate of PEI with tris-ligand complex [Gd(H₂L)₂(HL)]⁴⁻.

Research trends of heavy metal removal from aqueous environments

Heavy metals are a threat against human health. During the last century, with increased industrial activities, many water resources have been contaminated by heavy metals. Meanwhile the number of scientific studies about removing these toxic substances from aqueous environments has increased exponentially. According to bibliometric analysis the number of articles from 2000 to 2019 experienced a 1700% growth rate. China, India and the United States have published the greatest number of top-cited articles on the topic, with China in first place by a large margin. Six clusters of papers (by topic) were identified. From among the processes such as adsorption, membrane filtration, and ion exchange, adsorption has the lion's share of the investigations. Technical and efficiency considerations, as well as environmental impact and cost-effectiveness, were chosen as criteria to compare different methods. According to life cycle assessment, adsorption has the least amount of negative environmental effects compared to other trending methods such as membrane filtration and ion exchange. From a financial viewpoint, utilizing biosorbents and biochars for adsorption are the best options. Unlike other methods which depend on pretreatment processes and have a high energy demand, these sorbents are cost-effective and exhibit acceptable performance.

Highly selective removal of Ni(II) from plating rinsing wastewaters containing [Ni-xNH3-yP2O7]n complexes using N-chelating resins

Inorganic complexants, such as ammonia (AA) and pyrophosphate (PP), are often present alongside heavy metal ions in alkaline plating rinsing wastewater. We investigated the removal capacity of Ni(II) from waters containing [Ni-xNH₃-yP₂O₇]ⁿ complexes by chelating and ion-exchange resins in sole and dual-ligand systems. D463 (containing iminodiacetic groups) and PAMD (possessing polyamine groups) exerted superior performance under all conditions. Ni(II) adsorption on D463 decreased with AA and PP by 10.3% and 64.4%, respectively. Conversely, the adsorption on PAMD increased by 57.3% and 75.8%, respectively. PAMD exhibited high selectivity toward anionic [Ni-PP] species over free PP. More Ni(II) was captured by PAMD in the dual-ligand systems than sole systems, while the case for D463 was opposite. As confirmed by species tracking and DFT/XPS analyses, complexes breaking-Ni²⁺ capture was the dominant mechanism for D463, while the dual-site (non-charged and protonated amines) interactions with NiP₂O₇^2- on PAMD promoted its adsorption. The tandem combination D463-PAMD was the optimal mode to remove the most Ni(II). The actual wastewater test demonstrated that >210 BV effluent met the limit of 0.1 mg Ni(II)/L and the eluent contained 15 g Ni(II)/L. This study guides the application of chelating adsorption processes in the advanced treatment of plating rinsing wastewaters.

Adsorption of nickel (II) ions from wastewater using glutaraldehyde cross-linked magnetic chitosan beads: isotherm, kinetics and thermodynamics

Magnetic chitosan beads (MCSB), prepared from solution by using an external magnet, and the adsorption of Ni(II) ions from wastewater by MCSB and its cross-linked derivative with glutaraldehyde (GLU-MCSB) was investigated in an adsorption system. The GLU-MCSB sorbents are insoluble in aqueous acidic solution and improve adsorption capacity. The adsorption process was carried out by considering various parameters, viz. adsorbent dose, contact time, pH and temperature. Thermogravimetric analysis of beads shows that degradation takes place in two stages. Fourier transform infra-red spectra of magnetic beads exhibit an absorption band at 606 cm^-1 for Fe-O. The elemental analysis (energy dispersive X-ray analysis) and scanning electron microscopy were used to analyze the structure and characteristics of MCSB and GLU-MCSB. The Ni(II) removal efficiency attains a highest value of 95.12% with cross-linked GLU-MCSB in comparison to 79.5% with MCSB. Adsorption processes follow the pseudo-second-order rate kinetics model, which suggested that the rate-limiting step may be the chemical adsorption rather than the mass transport. The experimental data of adsorption fitted well with the Langmuir and Freundlich isotherms with a high correlation coefficient (R² > 0.9), showing that monolayer adsorption took place on the surface of GLU-MCSB absorbents. The negative values of entropy change, -175.64 and -163.30 J/(mol·K), and enthalpy change, -54.75 and -49.58 kJ/mol, for MCSB and GLU-MCSB suggest that the process is spontaneous and exothermic in nature.

Nano-manganese oxides-modified biochar for efficient chelated copper citrate removal from water by oxidation-assisted adsorption process

Removal of chelated copper from wastewater is more difficult than that of copper ions owing to its stable structure, wide range of pH tolerance, and stronger mobility. Copper citrate (CuCA) widely exists in the water system and inevitably poses serious hazards to human health and environment. Biochar as economic functional material has been widely used for environmental applications, especially in wastewater treatment. This study focused on the performance of manganese oxide-modified biochar (BC-MnO_x) toward uptake and removal of CuCA and to understand the related mechanism. The result indicated that the CuCA removal efficiency reached up to 99%. High removal efficiency and low concentration of dissolved Mn over a wide pH range proved that the BC-MnO_x is efficient and chemically stable. Furthermore, the removal mechanism may involve the following processes: First, CuCA was removed via the chemical bonds formed between CuCA and MnO_x on the surface of BC. Second, chemisorption due to the oxygen-containing functional groups or physisorption of porous structure in BC worked synergistically on CuCA. Third, CuCA was partially oxidized into low molecular weight acids by means of MnO_x, while the released Cu ions were retained on the adsorbent surface. This study demonstrates that BC-MnO_x is a promising material for the removal of CuCA from wastewater.

Removal of chelated heavy metals from aqueous solution: A review of current methods and mechanisms

Water contamination with heavy metal ions and organic compounds such as citrate, ethylenediaminetetraacetic acid, tartrate, pharmaceuticals, surfactants and natural organic matter, is a serious problem in the natural environment. Although many methods have been effectively applied to the removal of heavy metal complexes from aqueous solution, there is a lack of information available on the mechanisms, advantages and disadvantages of these various methods. This review summarizes the various treatment methods applied to the removal of heavy metal complexes, with a summary of the mechanisms of action and recent research progress. The methods reviewed in detail include electrolysis, membrane separation, adsorption, precipitation, replacement-coprecipitation, TiO₂ photocatalysis and Fenton oxidation-precipitation, with the advantages and disadvantages of each method discussed. Furthermore, the heavy metal complex removal mechanisms are analyzed comprehensively. Results show that the adsorption method exhibited unique merits, showing much promise for future development. Finally, this review comprehensively analyzes future prospects and developments in methods for removal of chelated heavy metals.

Nanoconfined Hydrated Zirconium Oxide for Selective Removal of Cu(II)-Carboxyl Complexes from High-Salinity Water via Ternary Complex Formation

Toxic metals are usually present as organic complexes in high-salinity effluents from various industries. The efficient removal of such metal complexes is an imperative but still challenging task due to their stable structure and high mobility. Herein, we propose a new strategy to remove Cu-carboxyl complexes from high-salinity water by using a commercially available nanocomposite HZO-201, i.e., nanohydrated zirconium oxide (HZO) confined inside anion exchanger D201. In contrast to D201 and a cation exchanger D001, which both adsorb Cu-citrate negligibly, HZO-201 exhibits preferable adsorption toward Cu-citrate (∼130 mg Cu/g-Zr) at high salinity (1.5 wt % NaCl). On the basis of scanning transmission electron microscopy energy-dispersive spectrometry (STEM-EDS), attenuated total reflection Fourier transform infrared (ATR-FTIR), and X-ray photoelectron spectrometry (XPS) analysis, the formation of ternary complex among Cu(II), citrate, and the embedded nano-HZO is evidenced to be responsible for the removal of Cu-citrate. The exhausted HZO-201 can be regenerated with a binary HCl-NaCl solution for repeated use for 5 cycles without capacity loss. Fixed-bed adsorption demonstrates that HZO-201 column is capable of producing ∼1150 bed volume (BV) clean water (<0.5 mg Cu/L) from simulated high-salinity wastewater, whereas only ∼10 BV and ∼60 BV was produced for the D001 and D201 columns, respectively. Furthermore, HZO-201 shows excellent removal of Cu(II) complexes with three other carboxyl ligands (oxalate, tartrate, and succinate).

Sequestration of chelated copper by structural Fe(II): Reductive decomplexation and transformation of Cu(II)-EDTA

Chelated coppers, such as Cu(II)-EDTA, are characteristically refractory and difficult to break down because of their high stability and solubility. Cu(II)-EDTA sequestration by structural Fe(II) (Fe(II)) was investigated intensively in this study. Up to 101.21mgCu(II)/gFe(II) was obtained by Fe(II) in chelated copper sequestration under near neutral pH condition (pH 7.70). The mechanism of Cu(II)-EDTA sequestration by Fe(II) was concluded as follows: 3Cu(II)-EDTA+7Fe(II)+9H2O → Cu(0)↓+ Cu2O↓(the major product)+2Fe2O3·H2O↓+3Fe(II)-EDTA +14H(+) Novel results strongly indicate that Cu(II) reductive transformation induced by surface Fe(II) was mainly responsible for chelated copper sequestration. Cu(0) generation was initially facilitated, and subsequent reduction of Cu(II) into Cu(I) was closely combined with the gradual increase of ORP (Oxidation-Reduction Potential). Cu-containing products were inherently stable, but Cu2O would be reoxidized to Cu(II) with extra-aeration, resulting in the release of copper, which was beneficial to Cu reclamation. Concentration diminution of Cu(II)-EDTA within the electric double layer and competitive adsorption were responsible for the negative effects of Ca(2+), Mg(2+). By generating vivianite, PO4(3-) was found to decrease surface Fe(II) content. This study is among the first ones to identify the indispensible role of reductive decomplexation in chelated copper sequestration. Given the high feasibility and reactivity, Fe(II) may provide a potential alternative in chelated metals pollution controlling.

Citric Acid Enhanced Copper Removal by a Novel Multi-amines Decorated Resin

Cu removal by a novel multi-amines decorated resin (PAMD) from wastewater in the absence or presence of citric acid (CA) was examined. Adsorption capacity of Cu onto PAMD markedly increased by 186% to 5.07 mmol/g in the presence of CA, up to 7 times of that onto four commercial resins under the same conditions. Preloaded and kinetic studies demonstrated adsorption of [Cu-CA] complex instead of CA site-bridging and variations of adsorbate species were qualitatively illustrated. The interaction configuration was further studied with ESI-MS, FTIR, XPS and XANES characterizations. The large enhancement of Cu adsorption in Cu-CA bi-solutes systems was attributed to mechanism change from single-site to dual-sites interaction in which cationic or neutral Cu species (Cu²⁺ and CuHL⁰) coordinated with neutral amine sites and anionic complex species (CuL⁻ and Cu₂L₂²⁻) directly interacted with protonated amine sites via electrostatic attraction, and the ratio of the two interactions was approximately 0.5 for the equimolar bi-solutes system. Moreover, commonly coexisting ions in wastewaters had no obvious effect on the superior performance of PAMD. Also, Cu and CA could be recovered completely with HCl. Therefore, PAMD has a great potential to efficiently remove heavy metal ions from wastewaters in the presence of organic acids.

Simultaneous biodegradation of Ni-citrate complexes and removal of nickel from solutions by Pseudomonas alcaliphila

The objective of this study was to study the simultaneous biodegradation of Ni-citrate complexes and removal of Ni from solutions by Pseudomonas alcaliphila. Adding excess citrate to 1:1 Ni-citrate complexes promoted the degradation of the complexes and removal of Ni. The alkaline pH generated by the metabolism of excess citrate caused partial dissociation of citrate from the Ni-citrate complexes, allowing degradation, and the released Ni was removed through bioaccumulation and precipitation. Addition of Fe(3+) enhanced the degradation of Ni-citrate complexes and removal of Ni from solutions. The displacement of Ni from recalcitrant Ni-citrate complexes by Fe(3+) and subsequent biodegradation of the degradable Fe(III)-citrate complex resulted in complete metabolism of citrate. The almost complete removal of Ni (>98%) can be attributed to the combination of coprecipitation with Fe(3+), bioaccumulation and precipitation. P. alcaliphila potentially could be applied in the treatment of effluent containing Ni-citrate complexes.

Sorption behaviour of Co(II) and Cu(II) on chitosan in presence of nitrilotriacetic acid

Separation and isolation of radioactive cobalt ((60)Co), one of the main contributors towards the activity build up in nuclear reactors, is essential for radioactive waste volume reduction during nuclear reactor decontamination procedures. In this context, sorption of free and complexed Co(II), Cu(II) and nitrilotriacetic acid (NTA) on the biosorbent, chitosan was studied. A detailed investigation on the role of pH on sorption of Co(II), Cu(II) and NTA was done. Uptake capacities of the metal ions and NTA were measured within pH range of 2.0-7.0. At pH above 5, the NTA uptake capacities were found to be higher in presence of the metal ions than in their absence. Effect of NTA was found to be more pronounced on copper uptake than on cobalt uptake. Significant change in selectivity of chitosan towards metal ion uptake from NTA medium was observed with respect to change in pH. At pH 2.9, the uptake of cobalt was found to be more than that of copper, while the selectivity was reversed at pH 6.0. The respective selectivity coefficient (k(Co/Cu)) values were found to be 2.06 and 0.072.