Layer Diffusion Research Articles

Symbiotically Augmented removal of Congo red by polyaniline/cobalt sulfide/graphite composites

The presence of Congo red dye (CR) in industrial sewage causes a serious threat to the environment. Therefore, it is imperative to develop high-performance, low-cost functional materials to mitigate such issues. During past decades, polyaniline and its composites have been recognized as an emerging candidate to remove hazardous organic effluents from water. The present work demonstrates the successful elimination of CR from water in presence of newly synthesized graphite/cobalt sulfide/PANI-based ternary composites. Several morphological or physicochemical characterization tools were adopted to confirm the formation of the ternary composite and subsequent synergistic interaction between individual elements of the composites. The experimental results delineate that a maximum of ∼95.55% CR removal (%) was achieved after 120 min. Fast removal (∼5–10 min) of CR dye is observed for APS/Ternary composite system. From the fitted experimental data utilizing 1st or 2nd order rate kinetic models, it was observed that the adsorption induced degradation of CR dye and the process was chemisorptions in nature. Further, an intra-molecular diffusion model was also introduced that signifies both boundary layer diffusion or intraparticle diffusion phenomenon was responsible for CR removal. Furthermore, the cytotoxicity profile of the composite treated Congo red aqueous solution was evaluated when exposed to L929 fibroblast cells after 24 h or 72 h of exposure and the result deciphers the non-toxic nature of composite treated CR water.

The oxidation behavior of Mg-Er binary alloys at 500 ℃

Thermodynamics and kinetics of the oxidation behavior of magnesium-erbium (Mg-Er) alloys at 500 ℃ were studied. The observed protective oxidation behavior, which follows parabolic kinetics, can be attributed to the formation of compact Er2O3 surface layer and continuous dissolution and diffusion of Er from the matrix. The surface film of Mg-8.1Er (wt%) alloy was composed of four sublayers, with the double Er2O3 layers consisting of a compact fine-grain layer and an underlying coarse-columnar-grain layer providing the protective effect. A four-stage oxidation process composed of three transient reaction-controlled oxidation processes and followed by a diffusion-controlled oxides growth process is proposed.

Rigid-Flexible Combined Impeller Enhancement in Leaching of Phosphate Rock: a Kinetics Study.

Conventional rigid impellers are frequently used in the leaching process of phosphate rock, which often form a symmetrical flow field in the reactor, leading to a reduction in the leaching efficiency. In this work, a rigid-flexible combined impeller was applied to the leaching process of phosphate rock to increase the leaching efficiency. The effects of the reaction temperature (T), sulfuric acid excess coefficient (ε), liquid–solid ratio (L/S), agitation speed (N), and leaching time (t) on the leaching of phosphate rock were investigated, and based on this, the leaching kinetics was studied. The results indicated that under the optimum parameters of a reaction temperature of 353 K, a sulfuric acid excess coefficient of 1.15, a liquid–solid ratio of 4.0 mL/g, an agitation speed of 280 rpm, and a leaching time of 120 min, the leaching rate of phosphate rock using the rigid-flexible combined impeller reached 89.1%, which was 7.1% higher than that of the conventional rigid impeller under the same electric energy consumption. The leaching process complied with the unreacted core shrinking model, and the reaction rate was controlled by product layer diffusion. The apparent rate equation of the leaching process was 1 – 2X/3 – (1 – X)2/3 = 2.06 × 10–3[ε]1.375[L/S]1.273[N]0.748 exp(−19.03 × 103/RT)·t, and the apparent activation energy was 19.03 kJ/mol. The numerical simulation and analysis of the leaching residue showed that the system temperature in the rigid-flexible combined impeller system was homogenized, and the mixing effect of reactants was enhanced through the multiposition movement of the flexible connection piece in the axial direction, so that the reactants participated in the chemical reaction more efficiently.

Performance Enhancement of PEM Fuel Cells with an Additional Outlet in the Parallel Flow Field

The design of bipolar plates is critical for improving the performance of proton exchange membrane fuel cells (PEMFCs). In this research, a new additional outlet based on a PEMFC’s parallel flow field was proposed, and three different positions of outlet were designed on the target side of gas flowing in parallel channels. The results revealed that the additional outlets are able to increase the gas speed through channels near the additional outlets, which results in a lower water saturation and a more uniform distribution of oxygen concentration at the interface between the catalyst layer (CL) and gas diffusion layer (GDL). With the variation of the outlet position in the target side, it was found that the additional outlet set in the middle of the target side exhibits the highest increase of peak power density, namely, 13%. Furthermore, the optimal position of the additional outlet was proved to be suitable for PEMFCs with various active surface areas, indicating the universality of the present results in the study.

Development of gum acacia based magnetic nanocomposite adsorbent for wastewater treatment

The present work reports development of a magnetic hydrogel adsorbent material based on the polysaccharide, gum acacia. This graft copolymer nanocomposite hydrogel of gum acacia and poly(3-chloro-2-hydroxypropylmethacrylate) has been made using microwave assisted technique. Magnetite nanoparticles have been generated by in-situ method and trapped in the network structure. The parent hydrogel and the hybrid nanocomposite were evaluated as adsorbents for sequestration of cationic dyes and metal ions from aqueous solution. The FTIR, XRD, SEM, TGA, VSM and BET studies have been performed to characterize the samples with respect to their physical and chemical structure, surface nature, thermal stability and magnetic property. The nanocomposite displayed super-paramagnetic nature owing to the presence of magnetite nanoparticles which also influenced the adsorption characteristics favourably as evident from the comparative studies performed with the non-magnetic parent gel. The nanocomposite exhibited larger surface area of 2.524 m2g−1 compared to the parent hydrogel with surface area of 0.481 m2g−1. The adsorption capability of the parent hydrogel towards methylene blue, rhodamine 6G, Cu(II) and Hg(II) are 224.4, 283.5, 282.4 and 275.3 mgg−1, respectively, were enhanced to 265.2, 344.4, 307.5 and 292.8 mgg−1, respectively, in the case of nanocomposite. The adsorption isotherm and kinetic studies indicated fit of Freundlich isotherm model and pseudo-second-order kinetic model, respectively. The adsorption has been shown to be a multistep process involving boundary layer diffusion and intra-particle diffusion playing major roles. The thermodynamic analysis manifested the adsorption as an exothermic and spontaneous process. The adsorption–desorption process studied in three cycles indicated excellent reusability characteristics of the nanocomposite making it a promising adsorbent material for wastewater treatment.

N-Doping of Graphene Aerogel as a Multifunctional Air Cathode for Microbial Fuel Cells

One of the main challenges faced by microbial fuel cells (MFCs) generating voltage is how to facilitate the oxygen reduction reaction (ORR) process using a specifically designed air cathode, especially by optimizing a three-phase catalytic interface and enhanced O2 diffusion on it. Herein, a three-dimensional porous N-doped graphene aerogel (NGA) is polymerized onto a steel mesh (SM) to construct a simple structure of an air cathode (NGA-x/SM) via hydrothermal synthesis and subsequent freeze-drying treatment; more specifically, NGA was simultaneously used as an efficient ORR catalyst layer and breathable gas diffusion layer to improve the performance of MFCs. In this system, the NGA-5/SM (with a precursor concentration of x = 5.0 mg mL-1) makes itself a perfect candidate to be used as an air cathode. Characterization parameters reveal that sub-micrometer micropores, defective multilayer structures, and the highest proportion of pyridinic-N (48.1%) exist in NGA-5/SM. Furthermore, electrochemical measurements demonstrate that it has an oxygen reduction peak potential of 0.63 V, a Tafel slope of 187 mV dec-1, and closest 4e- transfer pathway (n = 3.2-3.5). These data prove that a three-phase boundary can naturally form in NGA-5/SM, where the ORR occurs. More importantly, this work provides a proof of concept that a Pt-free air cathode could be prepared with high-efficiency NGA by a two-step preparation method to achieve a MFC maximum power density of 1593 mW m-2.

Pore Network Modeling of Hydrophilic / Hydrophobic Composite Microporous Layers for Polymer Electrolyte Fuel Cells

Liquid water accumulated in the catalyst layer (CL) and gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC, PEMFC) results in performance deterioration due to the inhibition of oxygen transport to the cathode CL (Flooding). To enhance the drainage in the GDL, a microporous layer (MPL) employed at the CL side of the GDL substrate is effective[1, 2]. Hydrophilic/hydrophobic composite MPLs in-house applied have mitigated the flooding [3], while the mechanism was not well clarified. A pore network model (PNM) [4, 5] of in-house hydrophobic MPLs constructed with focused ion beam scanning electron microscope (FIB-SEM) observations and validated with convective air permeation and oxygen diffusion measurements with changing wetting liquid saturation (Galwick, Porous Materials Inc., USA)[6] is modified for the analysis of the composite MPL.AcknowledgmentsThanks are offered to Professor Kohei Ito of Kyushu University for valuable discussions.

Efficient removal of crystal violet dye from aqueous solutions using sodium hydroxide-modified avocado shells: kinetics and isotherms modeling.

The main objective of this study is to optimize a new composite for the depollution of contaminated water. The sodium hydroxide-modified avocado shells (NaOH-AS) were firstly prepared, characterized by field-emission-scanning-electron-microscopy (FE-SEM), X-ray energy dispersive spectroscopy (EDS) and Fourier transforms infrared (FT-IR) spectroscopy, and applied for efficient removal of crystal violet dye (CV) in wastewater. In addition, the adsorption in a batch system of CV dye on the NaOH-AS material was studied. Therefore, we accomplished a parametric study of the adsorption by studying the effect of several important parameters on the decolorizing power of the used material, namely, initial pH, contact time, initial CV dye concentration, temperature, and the ionic strength effect on the CV dye adsorption process were systematically assessed. The highest adsorption efficiency of CV dye (>96.9%) by NaOH-AS was obtained at pH >8. The pseudo-second-order kinetic model gave the best description of the adsorption kinetic of CV dye on the AS and NaOH-AS adsorbents. In addition, the mass transfer of CV dye molecules from the solution to the adsorbent surface occurred in three sequential stages (boundary layer diffusion, intraparticle diffusion and adsorption equilibrium). The adsorption isotherm data were best fitted with the Freundlich model. The adsorption capacity of AS increased from 135.88 to 179.80 mg g-1 after treatment by 1 M NaOH. The thermodynamic study showed that CV dye adsorption onto NaOH-AS was an exothermic and feasible process. The electrostatic interactions acted as the only forces governing the CV adsorption mechanism. The NaOH-AS demonstrated a satisfactory reusability. Therefore, we can state that the as-developed NaOH-AS material has a potential application prospect as an efficient adsorbent for CV dye from wastewaters.

Investigation of species-mass diffusion in binary-species boundary layers at high pressure using direct numerical simulations

Direct numerical simulations of single-species and binary-species temporal boundary layers at high pressure are performed with special attention to species-mass diffusion. The working fluids are nitrogen or a mixture of nitrogen and methane. Mean profiles and turbulent fluctuations of mass fraction show that their qualitative characteristics are different from those of streamwise velocity and temperature, due to the different boundary conditions. In a wall-parallel plane near the wall, the streamwise velocity and temperature have streaky patterns and the fields are similar. However, the mass fraction field at the same location is different from the streamwise velocity and temperature fields indicating that species-mass diffusion is not similar to the momentum and thermal diffusion. In contrast, at the centre and near the edge of the boundary layer, the mass fraction and temperature fields have almost the same pattern, indicating that the similarity between thermal and species-mass diffusion holds away from the wall. The lack of similarity near the wall is traced to the Soret effect that induces a temperature-gradient-dependent species-mass flux. As a result, a new phenomenon has been identified for a non-isothermal binary-species system – uphill diffusion, which in its classical isothermal definition can only occur for three or more species. A quadrant analysis for the turbulent mass flux reveals that near the wall the Soret effect enhances the negative contributions of the quadrants. Due to the enhancement of the negative contributions, small species-concentration fluid tends to be trapped near the wall.

PEFCs Using Metallic Ti and Sn Electrocatalyst Supports

Carbon supports, which are widely used for electrocatalysts of polymer electrolyte fuel cells (PEFCs), can be electrochemically oxidized and corroded under the high potential conditions of PEFCs. Therefore, metal oxides, which are stable under PEFC conditions, have been proposed as alternative support materials. In this study, metals with porous structure, forming stable metal oxides on the outermost surface, are applied to electrocatalyst supports. Metallic titanium and metallic tin are examined, as their oxides (TiO2 and SnO2) are highly durable even under PEFC conditions. In addition, by tailoring porous structures, their metallic supports have the potential to function as both electrocatalyst layer and gas diffusion layer (GDL), simplifying the MEA structure and possibly fabrication processes. Such catalyst-integrated electrodes are prepared, and their electrochemical properties are evaluated.

Pore Network Modeling of Hydrophilic / Hydrophobic Composite Microporous Layers for Polymer Electrolyte Fuel Cells

Liquid water accumulated in the catalyst layer (CL) and gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC, PEMFC) results in performance deterioration due to the inhibition of oxygen transport to the cathode CL (Flooding). To enhance the drainage in the GDL, a microporous layer (MPL) employed at the CL side of the GDL substrate is effective. Hydrophilic/hydrophobic composite MPLs in-house applied have mitigated the flooding, while the mechanism was not well clarified. A pore network model (PNM) of our in-house hydrophobic MPLs is modified for the analysis of the composite MPL.

In-Materio Reservoir Computing in a Sulfonated Polyaniline Network.

A sulfonated polyaniline (SPAN) organic electrochemical network device (OEND) is fabricated using a simple drop-casting method on multiple Au electrodes for use in reservoir computing (RC). The SPAN network has humidity-dependent electrical properties. Under high humidity, the SPAN OEND exhibits mainly ionic conduction, including charging of an electric double layer and ionic diffusion. The nonlinearity and hysteresis of the current-voltage characteristics progressively increase with increasing humidity. The rich dynamic output behavior indicates wide variations for each electrode, which improves the RC performance because of the disordered network. For RC, waveform generation and short-term memory tasks are realized by a linear combination of outputs. The waveform task accuracy and memory capacity calculated from a short-term memory task reach 90% and 33.9, respectively. Improved spoken-digit classification is realized with 60% accuracy by only 12 outputs, demonstrating that the SPAN OEND can manage time series dynamic data operation in RC owing to a combination of rich dynamic and nonlinear electronic properties. The results suggest that SPAN-based electrochemical systems can be applied for material-based computing, by exploiting their intrinsic physicochemical behavior.

Metal-insulator multilayer precursors for enhanced Sm2+ absorption and Tm2+ luminescence in SiAlO thin films prepared by magnetron sputtering

Silicon-aluminum-oxigen (SiAlO) coatings doped with Sm2+ and prepared by reactive magnetron co-sputtering of Si, Al, and Sm targets, are attractive for luminescence solar concentrator applications but suffer from the low absorption between 300 and 600 nm. This article reports that the main cause of low absorption is a high concentration of undesired Sm3+. This finding is supported by optical transmittance, photoluminescence emission and excitation characterization, and X-ray photoelectron spectroscopy data of the Sm's 3d5/2 edge. We present an alternative deposition process for obtaining Sm doped SiAlO layers with enhanced Sm2+ absorption by incorporating Sm through the use of multilayer thin-film precursors composed of metallic Sm and SiAlO layers. After thermal post-deposition treatments, diffusion and reaction of the metallic Sm layers with the SiAlO host results in coatings showing the characteristic 5d → 4f transitions of Sm2+ in the region between 250 and 600 nm which were not detectable in Sm-doped single layers. This same deposition strategy produces Tm doped SiAlO coatings with Tm2+‘s characteristic luminescence at 1132 nm when the SiAlO host is in the mullite composition region. The photoluminescence excitation spectrum of Tm2+ is compared to phosphor with similar composition and covers the range between 300 and 700 nm.

In vitro inhibition of Xanthomonas spp. by lemon-scented gum extracts

The aim of this work was to evaluate lemon-scented gum (Corymbia citriodora) extracts in inhibiting the in vitro growth of Xanthomonas spp. The treatments consisted of aqueous, infusion, alcoholic and boiling extracts, in addition to the control with antibiotics. The methodology used to assess the antagonism was the double layer diffusion. The extracts were tested at 50, 35 and 15% concentration. None of the extracts inhibited the growth of X. citri pv. glycines, only the antibiotic bacitracin. As for X. vesicatoria, it was observed that only the alcoholic extract of C. citriodora leaves inhibited its growth, with emphasis on the 35% alcoholic extract, in addition to the antibiotics streptomycin and bacitracin. The growth of X. campestris was inhibited by alcoholic and aqueous extracts, highlighted on 50% and 35% alcoholic and 50% aqueous extracts, as well as by streptomycin and penicillin. The aqueous and alcoholic extracts of lemon-scented gum showed potential in the in vitro control of Xanthomonas and new studies will be carried out with new concentrations and in vivo control of diseases caused by these bacteria.

Kinetic study of low-temperature sulfur dioxide removal reaction by sodium carbonate using random pore model.

An experimental investigation of low-temperature SO2 removal reaction by porous sodium carbonate was carried out via thermogravimetry. Further, mathematical modeling was employed based on the random pore model to study the kinetics of this reaction. The experiments were performed at various temperatures (100-250 °C) and different SO2 concentrations (0.13-1.12 vol%). The initial slope method was used to determine dependency of the reaction rate constants versus temperature. First-order kinetics with respect to gaseous reactant was observed and the activation energy was determined as 22.5 kJ mol-1. Furthermore, product layer diffusivities were evaluated by comparison of experimental data with the model predictions. The mechanism was determined as competition between pore diffusion and surface reaction (mixed control regime) at initial stages, and product layer diffusion at later steps of the reaction. These random pore model predictions indicated good agreement with experimental conversion-time data under various conditions. The resulting kinetic parameters are essential for engineering calculations of SO2 removal from the coal-based power plants through low-temperature flue gas desulfurization.

Mass Transfer Principles in Column Percolation Tests: Initial Conditions and Tailing in Heterogeneous Materials.

Initial conditions (pre-equilibrium or after the first flooding of the column), mass transfer mechanisms and sample composition (heterogeneity) have a strong impact on leaching of less and strongly sorbing compounds in column percolation tests. Mechanistic models as used in this study provide the necessary insight to understand the complexity of column leaching tests especially when heterogeneous samples are concerned. By means of numerical experiments, we illustrate the initial concentration distribution inside the column after the first flooding and how this impacts leaching concentrations. Steep concentration gradients close to the outlet of the column have to be expected for small distribution coefficients ( L kg−1) and longitudinal dispersion leads to smaller initial concentrations than expected under equilibrium conditions. In order to elucidate the impact of different mass transfer mechanisms, film diffusion across an external aqueous boundary layer (first order kinetics, FD) and intraparticle pore diffusion (IPD) are considered. The results show that IPD results in slow desorption kinetics due to retarded transport within the tortuous intragranular pores. Non-linear sorption has not much of an effect if compared to values calculated for the appropriate concentration range (e.g., the initial equilibrium concentration). Sample heterogeneity in terms of grain size and different fractions of sorptive particles in the sample have a strong impact on leaching curves. A small fraction (<1%) of strongly sorbing particles (high ) carrying the contaminant may lead to very slow desorption rates (because of less surface area)—especially if mass release is limited by IPD—and thus non-equilibrium. In contrast, mixtures of less sorbing fine material (“labile” contamination with low ), with a small fraction of coarse particles carrying the contaminant leads to leaching close to or at equilibrium showing a step-wise concentration decline in the column effluent.

Trapping Rhodamine B dye using functionalized mango (Mangifera indica) pod

The use of acid-modified mango pod (AMMP) sorbent for removing Rhodamine B (Rh-B) dye from aqueous media was investigated. Raw mango pod (RMP) and AMMP sorbents were characterized using scanning electron microscopy (SEM), energy dispersive X-ray (EDX), powdered X-ray diffractogram (PXRD), Fourier transform infrared (FTIR), point of zero charge pH (pHpzc ), and Boehm titration (BT) techniques. Batch adsorption was employed to examine the influence of operational factors. Sorption kinetic parameters were calculated using pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models. The pseudo-second-order model best fitted the adsorption kinetic data most with maximum correlation coefficient (R2 > 0.99). The process of the adsorption was controlled by both boundary layer and intraparticle diffusion mechanisms. Four isotherm models (Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin) were utilized to analyze the equilibrium data at various temperatures. Freundlich model gave the best fit with the maximum regression (0.99), while the Langmuir isotherm model established a maximum monolayer adsorption capacity of 500 mg g-1 . Thermodynamic parameters studied revealed that the interaction is spontaneous and endothermic in nature. The cost analysis of the current study provides convincing proof that AMMP is efficient for removing Rh-B dye from solution by providing a saving of 225.2 USD/kg, which is eight times cheaper than commercial activated carbon. Consequently, the study revealed that AMMP is a viable, effective, and sustainable sorbent for Rhodamine B dye removal. PRACTITIONER POINTS: The powdered X-ray diffractogram (PXRD) showed the formation of new and intense peaks with the presence of highly organized crystalline structures on acid-modified mango pod (AMMP). Surface morphology of AMMP showed well-developed open surface pores required for effective adsorption of Rh B dye molecules. Economic feasibility of the present study showed that AMMP is more affordable than commercial activated carbon that costs USD 259.5/kg, thus translated to a saving cost of USD 225.2/kg and more than 7.5 times cheaper than the commercial activated carbon (CAC).

Synergetic strengthening mechanism of ultrasound combined with calcium fluoride towards vanadium extraction from low-grade vanadium-bearing shale

The effect and mechanism of ultrasound and CaF2 on vanadium leaching from vanadium-bearing shale were investigated systematically. In consideration of the enhancement for vanadium recovery, the combination of ultrasound and CaF2 (66.28%) exerts more evident effects than ultrasound (26.97%) and CaF2 (60.35%) alone, demonstrating the synergetic effect of ultrasound and CaF2. Kinetic analysis manifests that the product layer diffusion controls vanadium leaching in ultrasound system without CaF2, however product layer diffusion and interfacial reaction is the rate-controlling step for vanadium leaching in other three leaching systems. The combination of ultrasound and CaF2 notably decreases the activation energy (Ea) from 62.03 to 27.61 kJ/mol, nevertheless individual CaF2 only reduces the Ea to 50.70 kJ/mol. X-ray diffraction and fourier transform infrared spectrometer analyses show that the decomposition degree of the vanadium-bearing mica structure is the most significant in ultrasound and CaF2 system, proving the highest release degree of vanadium. Specific surface area and pore distribution combined with scanning electron microscope analyses reveal that the action of ultrasound and CaF2 would provide higher specific surface area, more abundant pores structure and cracks for the particles, which further prompts the rapid diffusion of H+, F− and HF, and achieves the conspicuous improvement of vanadium leaching recovery.

Enhanced permeability and photoprotective potential of optimized p-coumaric acid-phospholipid complex loaded gel against UVA mediated oxidative stress.

Photo-oxidative skin damage is mainly caused by the UV-A radiation of the sun. Synthetic sunscreens used to counter this acts mostly on the superficial skin layer and possess serious side effects. P-coumaric acid (PCA) is a UV-A protective plant phenolic having quick diffusion and distribution in superficial skin layers limiting its application as herbal sunscreen. The present study was designed to formulate an optimized phospholipid complex of PCA (PCAPC) through response surface methodology to enhance its skin permeation to deeper skin layers providing protection against photo-oxidative stress. PCAPC was characterized by FT-IR, DTA, PXRD, TEM, zeta potential etc. PCAPC was then incorporated into a gel formulation (PCAPC-GE) to facilitate its transdermal delivery. Physicochemical properties of the gel were assessed by pH, homogeneity, rheology, spreadability etc. In-vitro SPF and UVA-PF of the gel was evaluated and compared with conventional gel (PCA-GE). Ex-vivo skin permeation flux, permeability coefficient, skin deposition and dermatokinetic analysis were carried out to measure the rate and level of skin permeation. This was accompanied by in-vivo evaluation of PCAPC-GE and PCA-GE in the experimental rat model by measuring the various oxidative stress markers such as superoxide dismutase, catalase etc. PCAPC-GE provided high SPF and UVA-PF value compared to PCA-GE. The physicochemical parameters were suitable for transdermal application. PCAPC-GE enhanced the permeation rate of PCA by almost 6 fold compared to PCA-GE. Besides, a significant reduction of UV-A induced oxidative stress biomarkers were observed for PCAPC-GE. Thus, the PCAPC-GE may be an effective alternative of synthetic sunscreens due to its enhanced permeation and protection against UVA-induced oxidative stress.

Applicability of a novel and highly effective adsorbent derived from industrial palm oil mill sludge for copper sequestration: Central composite design optimisation and adsorption performance evaluation

Palm oil mill sludge (POMS) is a significant industrial waste generated in Malaysian palm oil mills. The re-utilisation of POMS as a value-added adsorbent for copper sequestration is highly desirable as a means for solid waste reduction. Therefore, this research aimed to evaluate the adsorption performance of POMS towards copper pollutant comprehensively. Adsorption attributes of the POMS-copper system were investigated using batch experiments, followed by statistical modelling to establish the simultaneous interactive effects of parameters and to optimise the system. POMS was determined to exhibit 9.72 point of zero charge, with a rough surface containing pore sizes within 1–7 µm and functional groups such as hydroxyl interacted with the copper. The statistically optimised adsorption capacity (15.84 mg g−1) was attained at 0.3 g POMS dosage, pH 4.56, 200 mg L−1 concentration and 60 min. The regeneration ability of POMS was satisfactory up to 5 adsorption-desorption cycles. Equilibrium and kinetic adsorption data were modelled based on the sum of normalised error (SNE) approach. The adsorption equilibrium was well represented by the Langmuir model (SNE: 3.69), with 16.56 mg g−1 Langmuir maximum adsorption capacity at 30 °C. Adsorption kinetics studies revealed that the pseudo-second-order kinetic (SNE: 2.98) was the most appropriate model, with copper ions chemisorbed onto POMS surface via multiple mechanisms such as boundary layer and intraparticle diffusion. The new POMS-copper system was thermodynamically exothermic (ΔH°: −12.72 to −4.25 kJ mol−1) and spontaneous (ΔG°: −19.07 to −11.44 kJ mol−1). Conclusively, POMS was an effective, low cost and eco-friendly adsorbent for the removal of toxic copper from aqueous environment.