Removal Of Organic Dye Pollutants Research Articles

Standalone Highly Efficient Graphene Oxide as an Emerging Visible Light-Driven Photocatalyst and Recyclable Adsorbent for the Sustainable Removal of Organic Pollutants.

Carbon-based nanostructures are promising eco-friendly multifunctional nanomaterials because of their tunable surface and optoelectronic properties for a variety of energy and environmental applications. The present study focuses on the synthesis of graphene oxide (GO) with particular emphasis on engineering its surface and optical properties for making it an excellent adsorbent as well as a visible light-active photocatalyst. It was achieved by modifying the improved Hummers method through optimizing the synthesis parameters involved in the oxidation process. This controlled synthesis allows for systematic tailoring of structural, optical, and surface functionality, leading to improved adsorption and photocatalytic properties for the sustainable removal of organic pollutants in water treatment. Several spectroscopic and microscopic characterization techniques, such as XRD, SEM, Raman, UV-visible, FTIR, TEM, XPS, BET, etc. were employed to analyze the degree of oxidation, surface chemistry/functionalization, morphological, optical, and structural properties of the synthesized GO nanostructures. The analyses showed excellent surface functionality with surface active sites for better adsorptive removal and a tunable band gap from 2.51 to 2.76 eV exhibiting excellent natural sunlight activity (>99%) for photocatalytic removal of the organic pollutant. Various adsorption isotherms have been studied with excellent adsorption capability (Qmax = 454.54 mg/g) as compared to the literature. The study introduces GO both as a proficient stand-alone (sole) nanoadsorbent as well as a nanophotocatalyst for the efficient removal of organic dye pollutants in water treatment. Additionally, the article highlights the sustainable solar light-induced green chemistry aspects of GO as an excellent recyclable adsorbent as a result of its self-cleaning ability under natural sunlight, demonstrating its potential in real eco-friendly environmental and practical applications.

Visible Light-Triggered Chitosan Supported Ternary Ferrite for Efficient Removal of Organic Dye Pollutants from Wastewater

Visible Light-Triggered Chitosan Supported Ternary Ferrite for Efficient Removal of Organic Dye Pollutants from Wastewater

Wurtzite CdS ratio tunability on α-Fe2O3/CdS synergistic heterostructure for enhanced UV-induced photocatalytic decomposition of rhodamine 6G dye pollutant

Hematite α-Fe2O3 exhibits a slow catalytic reaction rate and low performance in aqueous pollutant decomposition. In this work, α-Fe2O3/CdS binary heterostructure composites were created by introducing different ratios of wurtzite CdS into α-Fe2O3 within 0–100 wt% via precipitation and impregnation methods. The photocatalytic performance of α-Fe2O3/CdS was enhanced for the decomposition of aqueous rhodamine 6G (R6G) dye under ultraviolet (UV) irradiation. SEM images reveal that higher CdS loading results in increased interfacial contact between rod-like CdS and spherical-shaped α-Fe2O3. XRD and FTIR results confirm the coexistence of rhombohedral α-Fe2O3 and hexagonal CdS phases, with peak strength proportional to the ratio of component loading, as the respective elements were verified through EDX analysis. The UV-Vis spectroscopic analysis exhibits an enhancement in optical absorption and band gap broadening as the CdS loading rises. α-Fe2O3/CdS with increased CdS loading exhibits faster and more efficient R6G degradation under UV exposure, as evidenced by greater rate constants and degradation efficiency, with optimal CdS loading of 90 wt% achieving a maximum efficiency of 79.14%. The present heterostructure could be employed in future wastewater remediation for effective removal of organic dye pollutants, further promoting clean water preservation for global environmental sustainability.

Efficient scalable co-precipitated carbon dots and silver bromide doped manganese dioxide for catalytic and antimicrobial activity with evidential in-silico analysis

In this research, a co-precipitation approach was used to synthesize manganese oxide (MnO2) nanorods (NRs) doped with varying concentrations (2, 4 wt.%) of silver bromide (AgBr) and fixed amounts of carbon dots (CDs). CDs exhibit high photostability and a high electron transfer rate. AgBr can improve the activity of metal oxide owing to its electron-trapping effect. The new characteristics of as-prepared AgBr/CDs doped MnO2 NRs have a larger surface area, charge transport capabilities, and lower recombination chances, associating the synthesized material as a suitable choice for environmental cleanup. The removal of organic dye pollutants from wastewater and the elimination of harmful bacteria by the use of antibacterial activity were the goals of this research. The NRs demonstrated outstanding catalytic activity (CA) against rhodamine B (RhB) dye degradation. AgBr/CDs-doped MnO2 nanocatalyst indicates efficient dye degradation (96.39 %) in an acidic medium relative to other basic and neutral media. The fundamental components for generating reactive oxygen species (ROS) are electrons and hole pairs responsible for killing pathogenic bacteria. Compared to ciprofloxacin the substantial inhibition zones against Escherichia coli (E. coli) bacteria were measured at 2.95 mm. In silico molecular docking study of AgBr/CDs-doped MnO2 NRs proposed as promising DNA gyrase E. coli inhibitors.

A sulfonate-functionalized covalent organic framework for record-high adsorption and effective separation of organic dyes

The emerging global water crisis necessitates the development of solid adsorbents for the removal of organic pollutants from wastewater. Herein, we develop an anionic sulfonate-functionalized covalent organic framework (COF), termed TpStb-SO3Na, by the Schiff-base condensation between 2,4,6-triformylphloroglucinol (Tp) and 4,4′-diaminostilbene-2,2′-disulfonic acid (Stb-SO3H) under solvothermal conditions in the presence of sodium acetate. Thanks to the dense sulfonate groups in the periodic one-dimensional channels, the negatively charged TpStb-SO3Na displayed rapid kinetics (average ∼ 99% uptake in merely 10 min) and ultrahigh adsorption capacities for multiple cationic dyes including methylene blue (MB, 1078 mg g−1), crystal violet (CV, 1861 mg g−1), malachite green (MG, 5857 mg g−1) and janus green B (JGB, 1339 mg g−1) at room temperature. Noteworthy, the adsorption capacities of TpStb-SO3Na toward CV, MG, and JGB were the record-high values among the documented COFs, metal–organic frameworks (MOFs), and other benchmark adsorbents. In addition, the adsorption behaviors of MB, CV, MG, and JGB on TpStb-SO3Na obeyed the pseudo-second-order adsorption kinetic model. On the contrary, TpStb-SO3Na presented low adsorption capacity for anionic dyes like methyl orange (MO) and fluorescein sodium (FS), rendering it a promising sorbent for the separation of cationic and anionic dyes based on charge difference. Furthermore, TpStb-SO3Na was easily regenerated and reused for five cycles with no obvious loss in its adsorption performance. This work uncovers the vast potential of anionic COFs for the effective removal of organic dye pollutants in wastewater.

Insight into the synergistic effect of adsorption and photocatalysis for the removal of organic dye pollutants by novel BiFeO3@GO fibers

Insight into the synergistic effect of adsorption and photocatalysis for the removal of organic dye pollutants by novel BiFeO3@GO fibers

Cross-Linked Carboxymethylcellulose Adsorbtion Membranes from Ziziphus lotus for the Removal of Organic Dye Pollutants

The goal of this study is to assess Ziziphus lotus's potential for producing carboxymethylcellulose adsorption membranes with the ability to adsorb methyl green from wastewaters by the revalorization of its cellulosic fraction. The cellulose from this feedstock was extracted by an alkaline process and TAPPI standard technique T 203 cm-99 and afterwards they were carboxymethylated. The obtained carboxymethylcelluloses were deeply characterized, being observed that the carboxymethylcellulose produced from the alkaline cellulose presented the higher solubility due to its lower crystallinity degree (53.31 vs. 59.4%) and its higher substitution degree (0.85 vs. 0.74). This carboxymethylcellulose was cross-linked with citric acid in an aqueous treatment in order to form an adsorption membrane. The citric acid provided rigidity to the membrane and although it was hydrophilic it was not soluble in water. By evaluating the potential of the produced membrane for the removal of pollutant dyes from wastewater, it was observed that the adsorption membrane prepared from the carboxymethylcellulose's produced from the Ziziphus lotus was able to remove 99% of the dye, methyl green, present in the wastewater. Thus, this work demonstrates the potential of the Ziziphus lotus for the production of a novel and cost-effective carboxymethylcellulose adsorption membrane with high capacity to treat wastewaters.

Synthesis and Structure of 3D Bis[5‐(2‐pyridyl)tetrazolato]diquacopper(II) Framework: One‐pot Facile Fabrication of a Series of Catalysts Towards Dye Degradation in Sunlight

AbstractA copper‐based metal‐organic framework (MOF) Bis[5‐(2‐pyridyl)tetrazolato]diaquacopper(II) [{CuP}n], has been designed and structurally characterized by single‐crystal X‐ray crystallography. The three‐dimensional crystal structure belongs to the monoclinic space group P21/c with Z=4. Further a series of isostructural complexes [{Cu1‐xZnxP}n] (x=1, 0.75, 0.5, 0.25, 0), {P=(5‐(2‐pyridyl)tetrazolato)2.2H2O} have been synthesized via a facile one‐pot microwave irradiated solvothermal method. The optical and photocatalytic properties of the synthesized bimetallic complexes were investigated and compared with those of monometallic complexes. The effects of metal substitutions in the ligand backbone were investigated. A striking variation in the bandgap i. e., from 4.1 eV (x=1) to 2.79 eV (x=0) was observed among the catalysts with a minimum value of 1.93 eV (x=0.25). The optimized catalyst Cu0.75Zn0.25P exhibits the highest degradation activity and reached up to 99 % for RhB (rhodamine B) and 96 % for MB (methylene blue) in 120 min of Sunlight, corresponding to the respective photocatalytic degradation rates of 0.0328 min−1 and 0.0301 min−1. The kinetics of the degradation processes were best statistically described by pseudo‐first‐order. The catalyst was also found to have sufficient stability and recyclability with promising applications in removal of organic dye pollutants.

Preparation of Novel ZnO/Cu2O Heterojunctions Composite Film by Codeposition Method and Their Enhanced Photocatalytic Performance Analysis

Refractory organic pollutants have caused widespread concern about their pollution of water environments. Photocatalytic oxidation technology is an effective way to remove organic pollutants. Photocatalyst is the components core of photocatalytic oxidation technology. The development of visible light responsive catalysts with high catalytic activity is of great significance for the removal of organic pollutants using sunlight as a light source. In this paper, a codeposition method is used to prepare ZnO/Cu2O composite film with a heterojunction structure in one step. The microstructure and photoelectric properties of the prepared ZnO/Cu2O composite film are characterized and analyzed, and its photocatalytic performance is evaluated. Compared with pure the Cu2O film, the composite film exposes more (111) crystal planes, and has a smaller impedance and a larger photocurrent and open circuit voltage value. These findings indicate that the ZnO/Cu2O composite film exhibits excellent photogenerated carrier separation and migration efficiencies. Among the prepared samples, M2 demonstrates the highest photocatalytic and recycling performance. The calculation of the band position shows that the Fermi level of the composite film exhibits a significant shift compared to that of the pure Cu2O film. The analysis shows that the decrease in the recombination probability of photogenerated carriers caused by the shift of the Fermi level and formation of an internal electric field is the main factor for the significant enhancement in the Cu2O photocatalytic performance. This article provides a novel method and idea to realize the efficient removal of organic dye pollutants in sewage.

Application of the novel pH‐catalytic‐magnetic tri‐functionalities augmented bead for removal of organic dye pollutants

AbstractA tri‐functionality magnetic nanoparticles (MNPs) are crucial for environmental remediation. This research focused on the synthesis of pH‐catalytic‐magnetic polymer beads for dye removal by using phase inversion technique. In 15 wt% of polyethersulfone (PES) polymer possessed the highest porosity (80%) was used in the synthesis of MNPs‐PES beads. The methylene blue (MB) removal efficiency of MNPs‐PES beads and PES beads was 72.94% and 62.67%, respectively. Similarly, the removal efficiency of methyl orange (MO) dye is higher for MNPs‐PES beads (38.56%) compared to PES beads (34.16%). Besides, the removal efficiency for MB dye by MNPs‐PES beads increased from 77.59% to 96.90% when solution pH increased from 3 to 9 while removal of MO dye increased from 61.14% to 74.35% when solution pH decreased from 9 to 3. Moreover, the removal efficiency for MB dye increased from 72.94% to 93.43% when H2O2 is added. Additionally, MNPs‐PES beads were recollected successfully using external magnet. These indirectly proved that the MNPs‐PES beads were responsive towards the tri‐functionalities of pH‐catalytic‐magnetic. Lastly, the adsorption of MB dye onto MNPs‐PES beads were fitted well to Langmuir isotherm model and pseudo‐second‐order model with both correlations R2 of more than 0.98.

In-situ investigation of dye pollutant adsorption performance on graphitic carbon nitride surface: ATR spectroscopy experiment and MD simulation insight

The adsorption performances on graphitic carbon nitride (g-C3N4) surface were investigated for organic dye pollutants by both experimental and calculation methods. For experimental investigation, adsorption thermodynamics and kinetics results were in-situ obtained and evaluated. With ΔGMB+=−31.78<ΔGMO−=−17.38kJmol−1 by Langmuir modeling, g-C3N4 showed superior adsorption spontaneity of MB+ >MO-. With linear and exponential modeling, g-C3N4 showed only adsorption process for MB+ but both diffusion and adsorption processes for MO-. For simulation insight, all MB+ molecules but only parts of MO- molecules were inclined to orient in parallel position at g-C3N4 surface after optimization during low concentration. And both MB+ and MO- molecules were inclined to orient in perpendicular position at g-C3N4 surface after optimization during high concentration. Combined with experimental and calculation results, a molecular-orientation and force-dominance mechanism adsorption model are proposed to explain the surface interaction processes between dyes and g-C3N4. Electrostatic interaction and π-π stacking interaction were revealed to dominate for MB+ adsorption, and π-π stacking interaction and van der Waals force were revealed to dominate for MO- adsorption. This work obtained ‘localized’ interfacial information and elucidated in-situ intermolecular interactions at g-C3N4 interface, which can provide fundamental basis for operation removal of organic dye pollutants by g-C3N4.

Insight into the Synergistic Effect of Adsorption-Photocatalysis for the Removal of Organic Dye Pollutants by Cr-Doped ZnO.

The adsorption of dye molecules is an important process for the photodegradation removal of dye pollutants. In this work, a semiconductor photocatalyst of Cr-doped ZnO nanorods (Cr-ZnO NRs) was synthesized, and its adsorption-photocatalysis synergy (APS) effect was investigated for anionic methyl orange (MO-) and cationic methylene blue (MB+). The detailed thermodynamic information (including adsorption maximum capacity qmax, adsorption equilibrium constant Kads and adsorption efficiency AE %) and dynamic information (including adsorption rate constant ka, degradation rate constant kd and degradation efficiency DE %) were obtained to evaluate the different reaction performances for MO- and MB+. With qmax(MB+) = 40.59 mg g-1 > qmax(MO-) = 15.95 mg g-1, ka(MB+) = 20.61 min-1 > ka(MO-) = 4.62 min-1, and AE(MB+) = 40% > AE(MO-) = 9%, Cr-ZnO NRs showed much superior adsorption performance for MB+ than MO-. With kd(MB+) = 0.0430 min-1 > kd(MO-) = 0.0014 min-1 and DE(MB+) = 98% > AE(MO-) = 20%, Cr-ZnO NRs also showed much superior photodegradation performance for MB+ than MO-. The APS mechanism of Cr-ZnO NRs is revealed to be multiple π-π interactions and stronger electrostatic attractions dominant for enhanced adsorption of MB+ and higher AE and more photocatalytic active species dominant for enhanced photodegradation of MB+. The APS was furthermore characterized and verified by zeta potential analysis, Fourier transform infrared investigation, and fluorescence imaging. The results indicate that Cr-ZnO NRs are promising adsorbent and photocatalyst candidates favorable for positive MB+ than negative MO-. Such an APS investigation can effectively help to improve the photodegradation treatment performance of photocatalysts for dye pollutant removal.

Melt quenched V2O5/BiVO4 composite: A novel and promising adsorbent and photocatalyst

A large number of reports have been published showing negligible adsorption and promising photocatalytic properties of BiVO4 material. In this report, we have used a new and novel method for the fabrication of BiVO4, which can show significant adsorption alongwith the promising photocatalytic properties. V2O5/BiVO4 composite was synthesized using melt-quenching technique followed by the specific heat-treatment at 450 °C for 15 h. The X-ray diffraction (XRD) and Raman spectroscopy techniques confirmed the presence of monoclinic BiVO4 and orthorhombic V2O5 phases in heat-treated sample. The scanning electron microscope (FE-SEM) showed the formation of rod shaped crystals of 50–300 nm sizes and the transmission electron microscope (TEM) showed the heterojunctions formation between V2O5 and BiVO4 in the heat-treated sample. The heat-treated sample adsorb ~ 84% of MB dye from the MB dye solution of 11.75 mg/L initial concentration within 80 min under dark. The adsorption was mainly due to the presence of V2O5 present in it. In addition to this, the 70% of the adsorbed MB dye was recovered during desorption at room-temperature using the heat-treated sample, which is not possible with V2O5 material alone. The formation of heterojunctions between V2O5 and BiVO4 made desorption possible. Moreover, during photocatalysis experiment, the heat-treated sample successfully degraded ~ 88% of MB dye from the MB dye solution of 11.75 mg/L initial concentration within 4 h under ultraviolet (UV) irradiation due to the presence of photocatalytic active crystals of BiVO4 in it. Thus, the adsorption and photocatalytic capabilities of melt-quench V2O5/BiVO4 composite made it a promising material for the removal of organic dye pollutants from the waste water coming from textile industries.

Luminescent Triazene-Based Covalent Organic Frameworks Functionalized with Imine and Azine: N2 and H2 Sorption and Efficient Removal of Organic Dye Pollutants

Design of adsorbents with high stability and efficiency to achieve rapid uptake and high capacity for dye pollutants is a key challenge in environmental remedies. Herein, we have design and synthesized three triazene-based imine and azine functionalized covalent organic frameworks (COFs 1–3) via the formation of imine bonds by the condensation of a flexible tripodal tri aldehyde (TFPT) with linear diamines. The COFs are characterized by elemental analysis, FTIR, solid-state NMR, TGA, FESEM, TEM, and DRS. They exhibit permanent porosity with high surface areas and N2 uptake capacities of 540, 1600, and 2235 cc/g by COFs 1–3, respectively. They are found to be chemically and thermally stable and exhibit moderate hydrogen storage capacities. Photophysical studies reveal that COFs 1–3 exhibited low band gap values of 3.0, 2.5, and 2.25 eV, respectively, indicating their semiconducting nature. All three materials are highly luminescent, and their emission maxima are found to depend on the nature of the diamine...

Graphene Oxide/Co3O4 Nanocomposite: Synthesis, Characterization, and Its Adsorption Capacity for the Removal of Organic Dye Pollutants from Water.

In this work, graphene oxide/Co3O4 nanocomposite was synthesized via hydrothermal decomposition of [Co(en)3] (NO3)3 complex onto graphene oxide nanosheets. The as-prepared nanocomposite (denoted as GO/Co3O4) was structurally characterized by Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopies (TEM and SEM), energy dispersive X-ray (EDX) spectroscopy, magnetic measurements, and N2 adsorption-desorption analysis. The results demonstrated successful immobilization of Co3O4 nanoparticles with an average diameter size of around 12.5 nm on the surface of graphene oxide nanosheets. The adsorption performance of GO/Co3O4 nanocomposite was investigated towards different organic dyes in aqueous solutions. The results displayed that the adsorption rate of the GO/Co3O4 nanocomposite was 98% for methylene blue (MB) in 12 min, and 66% and 45% for Rhodamine B (RhB) and methyl orange (MO) in 40 min, respectively. The effects of various important parameters including adsorbent dosage, contact time, pH, and temperature on the adsorption process were investigated in detail. The equilibrium adsorption data were better fitted by Langmuir isotherm. Adsorption kinetics is well-modeled using pseudo-second-order model. Different thermodynamic parameters indicated that the adsorption process was physisorption and spontaneous. The findings of the present work highlighted facile fabrication of GO/Co3O4 and its application for rapid and efficient removal of MB from wastewater.

Eco-friendly and biocompatible cross-linked carboxymethylcellulose hydrogels as adsorbents for the removal of organic dye pollutants for environmental applications

ABSTRACTIn this study, new eco-friendly hydrogel adsorbents were synthesized based on carboxymethylcellulose (CMC, degree of substitution [DS] = 0.7) chemically cross-linked with citric acid (CA) using a green process in aqueous solution and applied for the adsorption of methylene blue (MB). Spectroscopic analyses demonstrated the mechanism of cross-linking through the reaction of hydroxyl functional groups from CMC with CA. These CMC hydrogels showed very distinct morphological features dependent on the extension of cross-linking and their nanomechanical properties were drastically increased by approximately 300% after cross-linking with 20% CA (e.g. elastic moduli from 80 ± 15 to 270 ± 50 MPa). Moreover, they were biocompatible using an in vitro cell viability assay in contact with human osteosarcoma-derived cells (SAOS) for 24 h. These CMC-based hydrogels exhibited adsorption efficiency above 90% (24 h) and maximum removal capacity of MB from 5 to 25 mg g−1 depending on the dye concentration (from 100 to 500 mg L−1), which was used as the model cationic organic pollutant. The adsorption of process of MB was well-fit to the pseudo-second-order kinetics model. The desorption of MB by immersion in KCl solution (3 mol L−1, 24 h) showed a typical recovery efficiency of over 60% with conceivable reuse of these CMC-based hydrogels. Conversely, CMC hydrogels repelled methyl orange dye used as model anionic pollutant, proving the mechanism of adsorption by the formation of charged polyelectrolyte/dye complexes.

Partition study of textile dye Remazol Yellow Gold RNL in aqueous two-phase systems

The removal of organic dye pollutants from wastewater produced by the textile industry is a complex problem that presents potential health risks to the general public. Remazol Yellow Gold RNL (YR) dye is readily used to dye cellulose base materials and the methods developed for its removal from aqueous systems are either inefficient or too expensive to be adopted by smaller textile manufactures. Our approach is based on aqueous two-phase system (ATPS) using salts and either polymers or ionic liquids to extract Remazol Yellow Gold RNL from wastewater. Parameters such as the nature of the electrolyte, molecular mass of polymer and tie line length (TLL) on the dye partition coefficient (KYR) were all evaluated. A phase diagram for the polyethylene glycol (PEG 4000gmol−1) and magnesiumsulfate system at 298.15K was obtained and used to study the partitioning of YR. The KYR values demonstrate the potential of both systems for the removal of dyes from industrial effluents. The partition mechanism was discussed based on the Haynes model and using the of Gibbs standard energy change (ΔtrG°). The driving force (enthalpy and/or entropy) that governs the partitioning of the dye depends on the nature of the ATPS. The optimized conditions that gave the best system was successfully applied to the removal of YR from wastewater obtained from a local textile manufacturer. The high KYR values in the presence of the effluent demonstrate the potential and robustness of the ATPS for the treatment of effluents from textile industries.

NiO hierarchical structure: template-engaged synthesis and adsorption property

NiO hierarchical structure was synthesized via a facile template-engaged solvothermal reaction of Mg5(CO3)4(OH)2·4H2O and Ni(NO3)2·6H2O, followed by annealing Ni-base precursor at high temperature in air. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and N2 adsorption-desorption. It has been demonstrated that the NiO hierarchical structure retains the morphology of the template, and possesses a high surface area (over 58 m2 g−1). Moreover, the growth mechanism of Ni-base precursor has been proposed. The porous hierarchical NiO exhibited good adsorption for the removal of organic dye pollutants from water. Thus, the porous hierarchical NiO is expected to find promising applications in catalysis, energy storage and sensors.

Removal of organic dye pollutants from wastewater by electrochemical oxidation

The removal of chemical oxygen demand (COD) and color from simulated dye wastewater containing organic dyes, direct yellow 4, C26H20N4O8S2Na2 and acid yellow 17, C16H10O7N4S2Cl2Na2 experimentally investigated using an electrochemical undivided cell reactor with Pt as anode and steel as cathode. Particular attention was paid to probe the effect of supporting electrolyte (NH4Cl), cell voltage, pH, and treatment time on the electrochemical treatment efficiency. Experiments were also carried out using 0.2 M NH4Cl as supporting electrolyte under the condition of free pH. The experimental results showed that the process could efficiently remove the color and COD from simulated dye waste water. The overall COD removal reached at 69.9% by using supporting electrolyte and increasing voltage. The process can be described by pseudo first-order kinetics for the removal of COD.