Synthesis of Acid Leached Diatomaceous Earth/ Amine-Functionalized Activated Carbon Composite and its Potential Application in Oily Wastewater Treatment
Due to the expanding worldwide oil exploration, a significant amount of oily wastewater is being produced. Oily wastewater lowers the diversity of aquatic life and plants by changing the structure of aquatic communities and food chains, among other ecological disturbances. Therefore, the overall aim of this project is to synthesize acid-leached diatomaceous earth (DE) and amine-functionalized activated carbon composite adsorbent for the treatment of oily wastewater. Acid leached diatomaceous earth was synthesized by leaching the raw diatomaceous step-wisely in a 120 mL volume of 2.2 M Nitric acid (HNO3) for three hours on a hotplate set to 60°C at a speed of 300 rpm. Thereafter, the mixture was filtered and the residue was washed with 1.5 L of deionized water. The residue was oven-dried at 50°C for 12 hours. The acid leached DE was then ground with mortar and pestle and then kept in a zip lock bag for further use in oily wastewater treatment. Activated carbon was synthesized from raw macadamia nutshell using of Orthophosphoric acid (H3PO4). Response surface methodology was used to determine the optimum conditions for synthesizing activated where 3 parameters (activation time, impregnation ratio, and activation temperature) were varied. Furthermore, 13 runs were completed at diverse conditions from the response surface methodology. To modify the synthesized activated carbon, hydroxylamine hydrochloride was used as a chemical agent to enhance the adsorption efficiency on the surface of the activated carbon and the amine-functionalized activated carbon was used in the removal of oil from oily wastewater. Lastly, a composite was prepared using both the synthesized acid leached diatomaceous earth and amine functionalized activated carbon and further evaluate its adsorption capacity and oil removal efficiency. All the synthesized adsorbents were applied in oily wastewater treatment and response surface methodology was used to optimize the variable parameters (adsorbent dosage, initial oil concentration and contact time) and to determine the optimum conditions for the oil removal efficiency. Therefore, this study was divided into 3 sections of results as discussed below. The first section of results is focused on the preparation of acid leached diatomaceous earth and its application in oily wastewater treatment. The results showed that acid treatment does not alter the physical structure of the diatomaceous earth significantly although there was an increase of silica content, its surface area, and pore volume from 78.98 to 91.57%, 15.02 to 22.40 m2/g, and 60.88 to 68.71 Å, respectively. The maximum adsorption capacity of 124.16 mg/g and oil removal efficiency of 78.55% were achieved at initial oil concentration of 3229.69 using 0.05 g adsorbent dosage and 119.20 mins contact time. The adsorption kinetics data fitted better to pseudo-second order model (R2=0.98) of reaction kinetics indicating that adsorption occurred through chemisorption. The adsorption isotherms were descried better by the Freundlich isotherms model (R2=0.87) indicating that adsorption occurred in a heterogeneous and multilayer surface. The second section focused on the synthesis of amine functionalized activated carbon for oily wastewater treatment. Activated carbon was prepared from macadamia nutshells (MNS) using H3PO4 as activating agent and further modified with amine groups using hydroxylamine hydrochloride. Amine-functionalization increases the adsorption capacity and removal efficiency due to expansion of the adsorbent’s particles. The oil removal experiment using amine-functionalized activated carbon showed a maximum percentage removal of 82.93% with adsorption capacity of 167.96 mg/g when initial oil concentration of 10 000 mg/L, adsorbent dosage of 0.1 g adsorbent dosage and contact time of 60 mins were used. The adsorption kinetics data fitted to pseudo-second order model (R2=0.92) of reaction kinetics indicating that adsorption occurred via chemisorption. The adsorption isotherm data fitted Freundlich adsorption isotherms (R2=1) model indicating that adsorption took place on heterogeneous and multilayer surface. The last section of the study focused on the preparation of the acid leached diatomaceous earth/amine-functionalized activated carbon composite adsorbent and its application in treatment oily wastewater. Due to challenges associated with synthesizing an ideal adsorbent for oily wastewater treatment, this composite adsorbent was synthesized to overcome the activated carbon drawbacks which include low thermal conductivity affecting the regeneration efficiency leading to a reduced adsorption capacity after being used for a certain period. Consequently, the advantages of the diatomaceous earth which include excellent heat resistance and its high porosity can improve the physicochemical properties such as surface area and thermal conductivity on the composite adsorbent. The SEM micrographs showed the presence of pores and rough surface with some particles of varying sizes on the surface of the composite material due to the addition of some functional groups which allowed the transfer of the oil molecules into the inner surface of the composite adsorbent. The prepared composite adsorbent showed a maximum oil removal percentage of 90.02% with maximum adsorption capacity of 416.67 mg/g at initial oil concentration of 5250 mg/L, an adsorbent dosage of 0.3 g, and contact time of 60 mins. The adsorption kinetics data showed a better fit to the pseudo-second order model (R2=0.98) showing that adsorption took place via chemisorption. Freundlich adsorption isotherm model descried the adsorption better showing that adsorption occurred on a heterogeneous and multilayer surface. Based on these findings, both materials prepared in this study has potential for use in treatment of oily wastewater. However, the composite displayed higher affinity towards treatment of oily wastewater compared to the acid leached diatomaceous earth and amine-functionalized activated carbon. Based on the conclusions, the main recommendations made from this study were to: 1) test the feasibility of adsorbent in field application and 2) conduct the cost-benefit analysis for this adsorbent.
Activated Carbon, Acid leached diatomaceous earth, Composite, Functionalization, Macadamia Nutshells, Oily Wastewater treatment
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