Solar photocatalytic removal of Cu(II), Ni(II), Zn(II) and Pb(II): speciation modeling of metal-citric acid complexes

Metal organic framework anchored onto biowaste mediated carbon material (rGO) for remediation of chromium (VI) by the photocatalytic process

Groundwater contaminated with hexavalent chromium Cr(VI) causes serious health concerns for the ecosystem. In this study, a hybrid amino functionalized MOF@rGO nanocatalyst was produced by utilization of a biowaste mediated carbon material (reduced graphene oxide; rGO) and its surface was modified by in situ synthesis of a nanocrystalline, mixed ligand octahedral MOF containing iron metal and NH2 functional groups and the prepared composite was investigated for Cr (VI) removal. The photocatalytic degradation of Cr(VI) in aqueous solutions was carried out under UV irradiation. Using a batch mode system, the effect of numerous control variables was examined, and the process design and optimization were carried out by response surface methodology (RSM). The photocatalyst, NH2-MIL(53)-Fe@rGO, was intended to be a stable and highly effective nanocatalyst throughout the recycling tests. XRD, SEM, EDS, FTIR examinations were exploited to discover more about surface carbon embedded with MOF. 2 g/L of NH2-MIL-53(Fe)/rGO was utilized in degrading 200 mg/L of Cr(VI) in just 100 min, implying the selective efficacy of such a MOF-rGO nanocatalyst. Moreover, the Eg determinations well agreed with the predicted range of 2.7 eV, confirming its possibility to be exploited underneath visible light, via the Tauc plot. Thus, MOF anchored onto biowaste derived rGO photo-catalyst was successfully implemented in chromium degradation.

Fluorescent starch-based hydrogel with cellulose nanofibrils and carbon dots for simultaneous adsorption and detection of Pb(II)

The adsorption removal of lead (Pb) ions has become a crucial area of research due to the potential health hazards associated with Pb contamination. Developing cost-effective adsorbents for the removal of Pb(II) ions is significantly important. Hence, a novel fluorescent starch-based hydrogel (FSH) using starch (ST), cellulose nanofibrils (CN), and carbon dots (CD) was fabricated for simultaneous adsorption and detection of Pb(II). A comprehensive characterization of FSH, including its morphological features, chemical composition, and fluorescence characteristics, was conducted. Notably, FSH exhibited a maximum theoretical adsorption capacity of 265.9 mg/g, which was 13.0 times higher than that of pure ST. Moreover, FSH was employed as a fluorescent sensor for Pb(II) determination, achieving a limit of detection (LOD) of 0.06 μg/L. An analysis was further performed to investigate the adsorption and detection mechanisms of Pb(II) utilizing FSH. This study provides valuable insights into the production of a novel cost-effective ST-based adsorbent for the removal of Pb(II) ions.

Treatment of electroplating industry wastewater: a review on the various techniques

Water pollution by recalcitrant compounds is an increasingly important problem due to the continuous introduction of new chemicals into the environment. Choosing appropriate measures and developing successful strategies for eliminating hazardous wastewater contaminants from industrial processes is currently a primary goal. Electroplating industry wastewater involves highly toxic cyanide (CN), heavy metal ions, oils and greases, organic solvents, and the complicated composition of effluents and may also contain biological oxygen demand (BOD), chemical oxygen demand (COD), SS, DS, TS, and turbidity. The availability of these metal ions in electroplating industry wastewater makes the water so toxic and corrosive. Because these heavy metals are harmful to living things, they must be removed to prevent them from being absorbed by plants, animals, and humans. As a result, exposure to electroplating wastewater can induce necrosis and nephritis in humans and lung cancer, digestive system cancer, anemia, hepatitis, and maxillary sinus cancer with prolonged exposure. For the safe discharge of electroplating industry effluents, appropriate wastewater treatment has to be provided. This article examines and assesses new approaches such as coagulation and flocculation, chemical precipitation, ion exchange, membrane filtration, adsorption, electrochemical treatment, and advanced oxidation process (AOP) for treating the electroplating industry wastewater. On the other hand, these physicochemical approaches have significant drawbacks, including a high initial investment and operating cost due to costly chemical reagents, the production of metal complexes sludge that needs additional treatment, and a long recovery process. At the same time, advanced techniques such as electrochemical treatment can remove various kinds of organic and inorganic contaminants such as BOD, COD, and heavy metals. The electrochemical treatment process has several advantages over traditional technologies, including complete removal of persistent organic pollutants, environmental friendliness, ease of integration with other conventional technologies, less sludge production, high separation, and shorter residence time. The effectiveness of the electrochemical treatment process depends on various parameters, including pH, electrode material, operation time, electrode gap, and current density. This review mainly emphasizes the removal of heavy metals and another pollutant such as CN from electroplating discharge. This paper will be helpful in the selection of efficient techniques for treatment based on the quantity and characteristics of the effluent produced.

Highly TEMPO-oxidized cellulose for removal of ionic and complexed cadmium from a complicated water system

TEMPO-NaDCC-oxidized cellulose (TNOCS) with a large surface area and an abundance of carboxyl groups was used to remove heavy metal ions (Cd, Cu, and Pb) and their organic acid complexes [HM-OAs] (OAs, i.e., citric acid (CA) and propionic acid (PA)), and then reveal their adsorption behaviors. Taking Cd and CA as examples, the results showed that some of Cd ions were first adsorbed onto TNOCS, and then, the existence of [Cd-CA^-] complexes formed a coordinated structure with preloaded Cd ions to serve as a bridge for combining TNOCS and [Cd-CA]. The maximum adsorption capacities of TNOCS for Cd and Cd-CA were 16.50 and 22.15 mg/g, respectively. Moreover, adsorption energies and molecular orbital distributions indicated that the adsorption capacity of TNOCS for [Cd-CA] was better than that for Cd alone. TNOCS can maintain greater than 90% adsorption capacity in five times regeneration experiments using EDTA, indicating that it is very efficient and stable. In addition, the electron density, deformation charge, and Mulliken charge distribution were confirmed that the electron transfer direction was from carboxyl groups to cadmium, whether it was cadmium ions or complexed cadmium.

A review on adsorptive separation of toxic metals from aquatic system using biochar produced from agro-waste

Water is a basic and significant asset for living beings. Water assets are progressively diminishing due to huge populace development, industrial activities, urbanization and rural exercises. Few heavy metals include zinc, copper, lead, nickel, cadmium and so forth can easily transfer into the water system either direct or indirect activities of electroplating, mining, tannery, painting, fertilizer industries and so forth. The different treatment techniques have been utilized to eliminate the heavy metals from aquatic system, which includes coagulation/flocculation, precipitation, membrane filtration, oxidation, flotation, ion exchange, photo catalysis and adsorption. The adsorption technique is a better option than other techniques because it can eliminate heavy metals even at lower metal ions concentration, simplicity and better regeneration behavior. Agricultural wastes are low-cost biosorbent and typically containing cellulose have the ability to absorb a variety of contaminants. It is important to note that almost all agro wastes are no longer used in their original form but are instead processed in a variety of techniques to improve the adsorption capacity of the substance. The wide range of adsorption capacities for agro waste materials were observed and almost more than 99% removal of toxic pollutants from aquatic systems were achieved using modified agro-waste materials. The present review aims at the water pollution due to heavy metals, as well as various heavy metal removal treatment procedures. The primary objectives of this research is to include an overview of adsorption and various agriculture based adsorbents and its comparison in heavy metal removal.

Citric acid enhanced phytoextraction of nickel (Ni) and alleviate Mentha piperita (L.) from Ni-induced physiological and biochemical damages

Phytoremediation is considered one of the well-established and sustainable techniques for the removal of heavy metals and metalloids from contaminated sites. The metal extraction ability of the plants can be enhanced by using suitable organic materials in combination with metal-tolerant plants. This experiment was carried out to investigate the phytoextraction potential of Mentha piperita L. for nickel (Ni) with and without citric acid (CA) amendment in hydroponic experiment. The experiment was performed in controlled glass containers with continuous aeration in complete randomized design (CRD). Juvenile M. piperita plants were treated with different concentrations of Ni (100, 250, and 500 μM) alone and/or combined with CA (5 mM). After harvesting the plants, the morpho-physiological and biochemical attributes as well as Ni concentrations in different tissues of M. piperita plants were measured. Results revealed that Ni stress significantly decreased the plant agronomic traits, photosynthesis in comparison to control. Nickel stress enhanced the antioxidant enzymes activities and caused the production of reactive oxygen species (ROS) in M. piperita. The CA treatment under Ni stress significantly improved the plant morpho-physiological and biochemical characteristics when compared with Ni treatments alone. The results demonstrated that CA enhanced the Ni concentrations in roots, stems, and leaves up to 138.2%, 54.2%, and 38%, respectively, compared to Ni-only-treated plants. The improvement in plant growth with CA under Ni stress indicated that CA is beneficial for Ni phytoextraction by using tolerant plant species. Graphical abstract.

Simultaneous and synergistic effect of heavy metal adsorption on the enhanced photocatalytic performance of a visible-light-driven RS-TONR/TNT composite

A hydrothermally synthesized rhodium/antimony co-doped TiO₂ nanorod and titanate nanotube (RS-TONR/TNT) composite was prepared for removal of heavy metals and organic pollutants from water under visible light irradiation. The composite provides the dual function of simultaneous adsorption of heavy metal ions and enhanced degradation of dissolved organic compounds. Acid treatment transformed titanate nanotubes to irregular tubular structures distributed homogeneously over untransformed RS/TONRs. Synergistic removal and degradation was studied with various heavy metals, Orange (II) dye, and Bisphenol A. The adsorption capacity of the composite for heavy metal ions was Pb(II) > Cd(II) > Cu(II) > Zn(II). The adsorbed metals enhanced photocatalytic degradation of the organic pollutants, but Cu was most effective, with degradation exceeding 70% for the dye and 80% for Bisphenol A after 5 h of treatment. Photocatalytic activity was enhanced more by adsorption than photodeposition of Cu ions. A decrease in XRD rutile peak intensity with adsorbed metal indicates a change in crystallinity which may enhance photocatalytic activity. Thick and bulging nanostructures in FE-SEM images signify ion adsorption within titanate pores. BET analysis indicated titanate nanotubes with adsorbed metal are mesoporous but their tubular structure persists. XPS showed more active Cu 2p_3/2 states under light, supporting an active role of Cu⁺ in photocatalytic ROS generation. Detection of ROS and Cu species using methanol, EDTA, pCBA, and benzoic acid probes provided strong evidence for degradation via a charge transfer mechanism. Findings demonstrate the potential of the RS-TONR/TNT composite for simultaneous removal of heavy metals and degradation of organic pollutants.

Recovery of nickel ions from wastewater by precipitation approach using silica xerogel

A facile route was developed to recover nickel ions from a synthetic wastewater. It involved the use of silica xerogel containing amine in the nickel sulphate solution resulting in the formation of a greenish precipitate. It was found that this precipitate was mostly amorphous Ni(OH)₂ spherical aggregate composed of nanosheets. The pH level of the solution was monitored, and it was maintained in the range of 10-10.5 due to the steady release of amine from the xerogel into the waste solution. The prepared silica xerogel would provide a stable environment for the chemical precipitation of metal ions in wastewater during the whole precipitation process. The silica xerogel was collected and reused for two more cycles of recovery. The nickel removal efficiencies (99.34˜99.65%) kept unchanged and higher than those reported earlier. The collected precipitate that contained nickel hydroxide with some residual silica could be utilized as glass colorant.

Efficient co-removal of copper and tetracycline from aqueous solution by using permanent magnetic cation exchange resin

This work aimed to examine a permanent magnetic cation-exchange resin (MCER) for synergistic co-removal of Cu(II) and tetracycline (TC) from their mixed solutions. Batch adsorption experiments and characterizations were performed to elucidate the adsorption mechanisms. The adsorption of Cu(II) followed the Langmuir isotherm model in most cases, while Freundlich isotherm model was more suitable for fitting TC adsorption, which was due to the surface coordination between adsorbed Cu(II) and TC and the formation of multilayer MCER-Cu-TC complexes. The equilibrium TC adsorption amount in binary Cu/TC system was about 5.5-13.5 times of that in sole system, whereas Cu(II) uptake was nearly unchanged. Decomplexing-bridging was ascribed as the primary mechanism, which involved the [Cu-TC] decomplexing and [resin-Cu] bridging for TC uptake. Moreover, these MCER microbeads could be reused with negligible loss in adsorption capacity during five adsorption-desorption cycles, indicative of great potential in synergistic co-removal of organics-Cu complexes from aqueous solutions.

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).

Effect of EDTA and citric acid on absorption of heavy metals and growth of Moso bamboo

The effect of EDTA and citric acid on accumulation, toxicity of heavy metals (Cu, Zn, Cd, and Pb), and growth of Moso bamboo was investigated in current experiment. The availability of heavy metals in soil and its uptake by plants has indicated toxicity. The results revealed that EDTA and citric acid has reduced biomass of Moso bamboo but non-significant difference in biomass was observed compared with control. Application of EDTA (10 mmol kg^-1) has significantly improved copper (Cu) by 56.5 and 84.9% in roots and above ground parts of plants. Application of EDTA (10 mmol kg^-1) has significantly enhanced lead (Pb) by 51.8 and 210.8% in roots and above ground parts of Moso bamboo. Furthermore, treatment of EDTA has significantly improved activities of water-soluble Cd, Cu, and Pb in soil by 98.9, 70.1, and 73.1 times compared with control. In case of contents of diethylenetriaminepentaacetic acid (DTPA)-extractable metals, the treatment of EDTA (10 mmol kg^-1) has produced maximum increase of 244.5 mg kg^-1 Zn and 157.9 mg kg^-1 Pb, respectively. It is concluded that effect of EDTA was superior compared with citric acid for improvement of phytoremediation potential of Moso bamboo.

Enhanced Photocatalytic Hydrogen Evolution from Transition-Metal Surface-Modified TiO2

This study describes the UV solution photodeposition of several earth-abundant 3d transition metals (Co, Ni, and Cu) onto the surface of nanoparticulate TiO2. Irradiated methanolic metal dichloride solutions with suspended Degussa P25-TiO2 (1-2 wt % metal to TiO2) yield visibly colored titanias, whereas the bulk TiO2 structure is unchanged; X-ray photoelectron spectroscopy confirms that metals are present on the titania surface in either reduced metal (Cu/Cu+) or metal cation states (Co2+ and Ni2+), and UV-vis diffuse reflectance spectroscopy shows new visible absorbance features. The analyzed bulk metal contents (∼0.04-0.6 at. %, highest for copper) are lower than the nominal metal solution content. Mixed-metal solution photodeposition reactions roughly parallel observations for single metals, with copper deposition being most favored. These 3d metal surface-modified titanias show significant (∼5-15×) improvement in UV photocatalytic H2 evolution versus unmodified TiO2. H2 evolution rates as high as 85 μmol/h (8500 μmol h-1 g-1) were detected for Cu-coated TiO2 using continuous monitoring of reactor headspace gases by portable mass spectrometry. Control experiments verify the necessity of the methanol sacrificial oxidant in both metal deposition and H2 evolution. In situ metal surface deposition is quickly followed by enhanced H2 evolution relative to TiO2, but at lower levels than isolated metal surface-modified titanias. The photodeposited 3d metal species on the TiO2 surface likely act to reduce electron-hole recombination by facilitating the transfer of photoinduced TiO2 conduction band electrons to protons in solution that are reduced to H2. This study demonstrates a facile method to modify photoactive TiO2 nanoparticles with inexpensive 3d transition metals to improve photocatalytic hydrogen evolution, and it shows the utility of quantitative real-time gas evolution monitoring by portable mass spectrometry.

Assessment of TiO2 photoactivity on the lead removal: kinetic and mechanistic processing

Removal of lead ions from aqueous solutions, in the presence and in absence of commercial TiO₂, under UV-light was studied. The influence of catalyst mass, concentration of Pb(II) ions and of citric acid in the starting solution as well as the impact of illumination conditions on the removal rate were also investigated. The results were fitted on the pseudo-first order, pseudo-second order and Elovich kinetic models, Weber-Morris intraparticle and liquid film diffusion models, in order to establish the photoreduction mechanism. The Pb removal rate increased in time by a combined linear-exponential mechanism. The catalyst had a positive influence on the removal rate at the start of the photoreduction. At a low concentration of Pb(II) ions (20 mg/L), the amount of photoreduced Pb(II) ions was proportional to the concentration of the citric acid solution, but at high concentrations (240 mg/L), the correlation was inversely proportional. The rate limiting steps in the removal of lead were both the photoreduction on the TiO₂ surface and the diffusion through the film surrounding the catalyst particle. The lead removal was significantly influenced by the mobility and by the concentration of the species in solution.

Titanium dioxide nanotube arrays with silane coupling agent modification for heavy metal reduction and persistent organic pollutant degradation

TiO₂ catalysts have gained extensive attention owing to their excellent catalytic ability, relatively stable physical and chemical properties and competitive price.

Synthesis of ZnWO4/CdWO4 core–shell structured nanorods formed by an oriented attachment mechanism with enhanced photocatalytic performances

We have described the oriented attachment mechanism in which CdWO₄ nanorods obviously act as an epitaxial ‘substrate’ and guide the ZnWO₄ aggregation process for the formation of CdWO₄ nanorod based aggregated structures.

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.

Enhanced photocatalytic reduction of aqueous Pb(II) over Ag loaded TiO2 with formic acid as hole scavenger

In the present study, photocatalytic Pb(II) reduction over TiO(2) and Ag/TiO(2) catalysts in the presence of formic acid was explored to eliminate Pb(II) pollution in water. Ag/TiO(2) catalysts were prepared by the photo-deposition method and characterized using UV-Vis diffuse reflectance spectra, X-ray reflection diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Ag deposition on TiO(2) led to enhanced photocatalytic Pb(II) reduction and the Ag/TiO(2) catalyst with a Ag loading amount of 0.99 wt.% exhibited the optimum photocatalytic activity. For Pb(II) reduction over Ag/TiO(2) with a Ag loading amount of 0.99 wt.%, initial Pb(II) reduction rate was found to be dependent on the initial concentrations of formic acid and Pb(II). Increasing initial Pb(II) concentration led to linearly increased initial Pb(II) reduction rate. At low formic acid concentration, in parallel, initial Pb(II) reduction rates increased with formic concentration, but remained nearly identical at high formic acid concentration. Solution pH impacted the photocatalytic Pb(II) reduction and after irradiation for 100 min Pb(II) was removed by 11.8%, 91.2% and 98.6% at pH of 0.8, 2.0 and 3.5, respectively, indicative of enhanced Pb(II) reduction with pH in the tested pH range. The results showed that Ag/TiO(2) displayed superior catalytic activity to TiO(2), highlighting the potential of using Ag/TiO(2) as a more effective catalyst for photocatalytic Pb(II) reduction.

Analysis of organic compounds in an urban wastewater treatment plant effluent

In the present work we fractioned the effluent water from an urban sewage treatment plant (USTP) of Ribarroja (Valencia, Spain) using the conventional protocol based on DAX8 and XAD4 resins. The fractions were analyzed by elemental analysis, FT-IR, 1H-NMR, COSY-NMR, HSQC-NMR, FAB+-MS and also by derivatization with bis(trimethylsilyl)trifluoroacetamide with 10% of trimethylchlorosilane. The four fractions obtained have common spectroscopic features and individual compounds indicating that the fractioning procedure is inefficient at separating different families of compounds. Gas chromatography/mass spectrometry (GC-MS) analysis of the derivatized fractions allowed identification of many individual compounds. The main classes of organic compounds present in the effluent are saccharides, amino acids, fatty acids, hydroxyacids, aromatic compounds and steroids. Also we were able to identify in the effluent the emerging pollutants paracetamol and ketoprofen - two best-selling antiinflammatory drugs used in humans.