Preparation of Activated Carbons and Activated Carbon-Silver Nanoparticles Composites from Ricinodendron heudelotti shells: Adsorption tests of Indigo Carmine and Methyl Orange Dyes from Wastewater and their Anti-Bacterial properties

Abstract

In the course of this present work, two Activated carbons from Recinodendron heudelotti shells (RHS) namely H3PO4 acid activated carbon (ACP) and ZnCl2 activated carbon (ACZ) were successfully produced using chemical activation with phosphoric acid (H3PO4) and zinc chloride (ZnCl2) as activating agents. The preparation of the two samples was modeled through Designing of Experiments (DoE) by using Central Composite Design (CCD). CCD design was used to elucidate the experimental domain both within and without the chosen domain of studies in order to achieve the best overall optimization of the process. This design has been used to develop model equations for activated carbon (AC) preparation by using statistical package Minitab 16 software. The interpretation of effect of main factors and their interactions were carried out and the developed models were validated by conducting experiments at the predicted conditions. The effect of activation temperature (T), activation time (t) and impregnation ratio (R) on iodine number (IN), methylene blue (MB) number and the percentage yield (%Y were studied. The optimum condition of the preparation of the ACP and ACZ samples were found to be 431.821°C, 39.546 min and 0.659 for T, t and R respectively for ACP and 591.506°C. 43.623 min and 1.050 for T, t and R respectively for ACZ. The proximate analysis was used to determine the macronutrients in RHS and was obtained with percentages of moisture content, ash content, volatile matter and fixed carbon to be 5 %, 5 %, 62.57 % and 27 % respectively. X-ray fluorescence result showed that RHS are composed mainly of calcium oxide (CaO) and magnessium oxide (MgO). FTIR analysis showed that hydroxyl, carbonyl, aliphatic carbon, ethers, alcohol, phenol and carboxylic groups present on the surfaces of the RHS and its activated carbons. From the ACP and ACZ samples, two composites samples were prepared by dopping ACP and ACZ with silver nanoparticles (AgNPs). These composites (ACP/AgNP and ACZ/AgNP) were made by successful precipitation loading onto ACP and ACZ with silver nanoparticles of the RHS aqueous extract (RHSNP). As in the case of ACP and ACZ, the two new materials were also characterised by scanning electron microscopy (SEM), Energy dispersive X-ray diffraction (EDX) measurements, Fourier Transform Infra-Red (FTIR) spectroscopy, X-ray diffraction (XRD), particle size measurements by Zeta sizer and specific surface area by BET. The SEM results shows spongy rock-like surface on all adsorbents with the presence of pores. EDX and XRD show the presence of crystalline zincite on ACZ and Ag on theACP/AgNP and ACZ/AgNP. The FTIR spectral for both composite adsorbents presume a composite material while the zeta sizer show that all the samples prepared were in the nanorange. The textural properties of the different materials (RHS, ACP, ACZ, ACP/AgNP and ACZ/AgNP) show an increase in the specific surface area from the raw precursor (RHS) was found to be 182 m2/g which increased to 386.613 m2/g in ACP and 615.400 m2/g in ACZ after preparation. The composites ACP/AgNP and ACZ/AgNP were found to have lower specific surface area with values of 367.400 m2/g and 335.100 m2/g in ACP/AgNP and ACZ/AgNP respectively than their pristine carbons. The antibacterial activities of the four samples, the RHSNP and the RHS extract was done by the Broth microdilution test method on seven different bacteria which are the typhoid-causing Salmonella Typhi, food-poisoning Staphylococcus aureus, pneumonia-causing Klebsiella pneumoniae as well as the diarrhealmanifesting Escherichia coli and Singehla flexneri species and also on Salmonella Enteritidis and Salmonella Typhimurium. The extract and ACs showed no antimicrobial activities while the antimicrobial properties were proven to be very interesting for the nanoparticles, ACP/gNP and ACZ/AgNP but higher for the ACZ/AgNP (7.812≤MIC≤ 31.25 μg/mL). Furthermore, the adsorption capacities of the ACZ and the ACZ/AgNP were investigated using the hazardous Indigo Carmine (IC) and Methyl Orange (MO) dyes. The equilibrium quantities adsorbed using the maximum equilibrium concentration were found to be 177.801 mg/g, 133.407 mg/g, 160.678 mg/g, and 89.181 mg/g for IC adsorption; 178.385 mg/g, 124.204 mg/g, 165.384 mg/g and 120.102 mg/g for MO adsorption on ACP, ACZ, ACP/AgNP and ACZ/AgNP respectively. This gives rise to a percentage decrease of 9.630 % and 33.151 % for IC adsorption; 7.273 % and 3.303 % for MO adsorption from ACP to ACP/AgNP and ACZ to ACZ/AgNP respectively. RHS is therefore a good and promising precursor for the preparation of activated carbon and nanoparticles for bacterial containing water purification and for the treatment of bacterial infections.