Enhanced Cadmium Removal from Aqueous Solutions by Immobilized Activated Carbon on Textile-Coated Inclined Plates
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Date
2024-09-30
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Addis Ababa University
Abstract
The rapid increase in population and industrial growth has led to the generation of large amounts of wastewater, which in turn has an impact on the water quality. Studies have shown that global increase in industrialization is one of the major contributors of heavy metals in wastewater. Common heavy metals found in wastewater include cadmium, chromium, arsenic, lead, mercury and nickel among others. These metals pose risks to aquatic ecosystems and human health, with potential effects including neurological, cancer and developmental issues. The strict regulations governing wastewater discharge require effective treatment techniques. Adsorption has gained popularity as one of the methods for water and wastewater treatment owing to its relative simplicity, efficiency and cost effectiveness. Adsorption for water and wastewater treatment has been widely studied over the past decade, and has mainly focused on the use of low-cost renewable adsorbents for pollutants removal. Activated carbon is considered as one of the favorable adsorbents for use in adsorption process due to its enhanced surface area. The use of activated carbon (AC) as an adsorbent in water and wastewater treatment has been limited to laboratory scale due to among other things the powdery form of most activated carbons which pose a number of limitations for industrial application. The use of powdered activated carbon (PAC) is hindered by such challenges as adsorbent-adsorbate separation after adsorption process leading to secondary pollution, and regeneration of the spent adsorbent material which leads to loss of the valuable adsorbent through leaching. Even though granular activated carbon is used to overcome these challenges, its use is only limited to batch and column adsorption configurations. These challenges, along with others discussed in this study have hindered the large-scale implementation of the adsorption technique in wastewater treatment systems. This study aimed at developing an innovative wastewater treatment process that integrates the inclined plate settlers (IPS) and Composite Adsorbent Coating (CAC) for heavy metal removal from aqueous solutions in a continuous set-up in an effort to boost the use of adsorption for large-scale industrial applications. The first part of this study focuses on the preparation of activated carbon from Prosopis juliflora (PJAC) wood by pyrolysis and chemical activation. The objective was to assess its effectiveness as an adsorbent for synthesizing CAC for heavy metal removal. The influence of impregnation ratio (IR), carbonization time (t), and carbonization temperature (T) on the Cd2+ percent (%) removal was evaluated using the Box-Behnken Design (BBD) of the Response Surface Methodology (RSM) (Design Expert software version 11). v The results indicated that all the variable preparation factors were significant (p< 0.05) in the Cd2+ removal by PJAC with T being the most significant (p < 0.0001). At the optimum conditions of IR=1.8, T=595 ºC and t=174 min, the model predicted a 99.9% Cd2+ removal efficiency while the adsorption experiment obtained a 96.7% removal efficiency, respectively. The SEM images of the optimized PJAC revealed a rough and porous morphological surface with an SBET of 600.4m2/g and a near neutral pHPZC of 6.92. These findings highlighted the potential of utilising invasive plants like Prosopis Juliflora as effective adsorbents for removing heavy metals. Later the prepared PJAC was then used to prepare the CAC to overcome the challenges associated with powdered activated carbon (PAC) in water and wastewater treatment. CAC was synthesized using a simple sol-gel method for the simultaneous reduction of Cd2+ and Cr2O72-. The CAC was also characterized by various techniques. Statistical analysis confirmed that pH and contact time significantly (p < 0.0001) affected both metal ions removal. Using the optimized conditions (pH=8.5, CAC dosage=0.25, adsorbate concentration=5mg/L, contact time=30 min and temperature=23.73°), the predicted and experimental ion removal efficiencies were 86.86 and 83.98% for Cd2+ and 94.26 and 58.08% for Cr2O72-, respectively. The Langmuir adsorption isotherm was the best-suited model (R2 > 0.99), while the metal ions removal was regulated by the PSO kinetic model (R2 > 0.999). The batch adsorption process was endothermic and spontaneous, as indicated by thermodynamic values (− ΔG°, +ΔH°, +ΔS°). The study only observed a 10% decline in the Cd2+ removal effectiveness of the CAC after three adsorption-regeneration cycles (with 0.1 M HCl of pH 0.3 as the eluent), indicating its stability for heavy metal removal. The final part of the research aimed at developing an innovative method for treating wastewater, an Inclined Plate Adsorber (IPA). The effects of angle of plate inclination (𝜃𝑝), influent flow rate (Q) and adsorbate initial concentration (Ci) on Cd2+ percent removal efficiency (%) were studied. At optimized operating parameters (θ=45˚, Q=5 ml/min and Ci=1.87 mg/L) the IPA Cd2+ predicted (R2=0.9926) and experimental removal efficiencies were 75.8% and 69.7±4.67%, respectively. The adsorption capacity (mg/g) of IPA calculated using the breakthrough curve analysis (BTCA) was 9.6 mg/g. Comparing IPA performance with a tank without plates and a tank with plain plates, the Cd2+ removal efficiencies were 2.4±0.1% and 4.6±1.1%, respectively, confirming that the dominant pollutant removal mechanism in an IPA system is adsorption. Additionally, breakthrough curves were acquired for various Q, Ci, and 𝜃𝑝. The results underpin the potential of using IPA for industrial wastewater treatment and enhancing the use of adsorption on a larger scale.
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Keywords
Adsorption, Breakthrough Curve Analysis, Composite Adsorbent Coating, Inclined Plate Settler, Optimization, Prosopis Juliflora, Synergistic Integration