Adsorption of Methylene Blue onto Chemically Prepared Activated Carbon from Date Palm Pits: Kinetics and Thermodynamics

Three activated carbons were prepared using phosphoric acid (P) as an activating agent from date palm pits (DPP) as a precursor via thermal pretreatment producing (CP212, CP214 and CP124) samples, where the ratio of raw material to phosphoric acid is (2:1and 1:2) respectively at curing time two days for first sample and four days for the second and third sample, the activating temperature was 550°C, the precursor was washed with distilled water, dried, crushed, and then sieved. In order to study the effect of phosphoric acid modification, the characteristics of the activated carbon produced were determined before and after acid modification and subsequently compared. These characteristics include surface morphology, surface area, average pore diameter and pore volume. Characterization results showed that modification of date palm pits with phosphoric acid enhanced the surface area of the activated carbon from 427.8 to620.3 m/g. The average pore diameter was also enhanced from 1.14 to 1.82 nm. SEM analysis confirmed the improvement in surface area and pore development resulting from the phosphoric acid modification. Introduction MB is synthetic thiazine dye of an amorphous nature with a molecular formula C16H18ClN3S.xH2O. It is also called basic blue, tetra methylthionine chloride and colour index (Cl) number 52012. The molecular weight of MB is 320 and its maximum wave length 662nm. It is dark green powder, with a characteristic deep blue colour in aqueous solution where it dissociates into an MB cation and a chloride anion dye, Methylene blue is a common dye mostly used by industries involve in textile, rubber, paper, plastics, leather, pharmaceutical cosmetics, and food industries. Effluents discharged from such industries contain residues of dyes. Consequently, the presence of very low concentrations in effluent is highly visible [1, 2]. Discharge of colored waste water without proper treatment can results in numerous problems such as chemical oxygen demand (COD) by the water body, and an increase in toxicity. Organic dyes are harmful to human beings the need to remove color from waste water become environmentally important. It is rather difficult to treat dye effluents because of their synthetic origins and mainly aromatic structures, which are biologically non-degradable. Moreover, their degradation products may be mutagenic and carcinogenic [3,4]. Many dyes may cause allergic dermatitis, skin irritation, and dyes function of kidney, liver, and brain, reproductive and central nervous system [5]. It is estimated that 10–15%of the dyes are lost in the effluent during the dyeing processes. Activated carbon has been extensively used in wastewater treatment, chemical By-Products of Palm Trees and Their Applications Materials Research Forum LLC Materials Research Proceedings 11 (2019) 275-285 doi: https://doi.org/10.21741/9781644900178-23 276 recovery and catalytic support industries primarily due to large surface area and presence of different pore sizes [6]. In most reported cases chemical activation is preferred as it, getting better porous, more surface area and high yielding activated carbons [7]. Date palm pits are considered one of the most useful and an abundant renewable agricultural waste, in 2006 world production of dates, was about 7 million tons. About 14% of the fruit is waste material, in the form of seeds. Date stones represent about 10% of the date weight. The chemical composition of date stone consists of hemicelluloses (23%), lignin (15%), and cellulose (57%) [8]. Date palm pits is not consumable by humans in any form; it has a high content of crude fiber (around 19%) that may cause digestibility problems in ruminant animals as well The main objective of this research is to prepare phosphoric acid (H3PO4)-activated carbons from date palm pits. The following methods were used to characterize the prepared activated carbons: nitrogen adsorption at -196°C, scanning electron microscopy for analysis of the surface chemistry, pHpzc, surface pH analysis, and measurement of MB adsorption capacity at different temperatures. Special attention was paid to kinetic studies. Materials and Methods 1. Materials All the primary chemicals used in this study were of analytical grade. Methylene blue with 99.99% purity was obtained from Sigma-Aldrich Company and phosphoric acid. Distilled water was utilized throughout the experiments for solution preparation and glassware cleaning. 2.Preparation of activated carbons In our study, we used (Sawi date palm pits) and the material was collected from pastry factory in (Shubra Al Khaimah). The date palm pits were washed with hot deionized water to remove dust and other impurities, and dried at105C.Raw materials were ground into fine particles and sieved to a particle size of (2mm). Phosphoric acid activated carbon samples were prepared by soaking date palm pits in 50 wt. % analytical grade phosphoric acid at room temperature at different ratios of raw material to phosphoric acid (CP212, CP214 and CP124) for curing time two days for the first sample and four days for the second and third sample .The slurry was occasionally stirred, the solid was then separated and heated gradually in absence of air in a stainless steel reactor at a rate of heating = 10 C /min up to 550 C and then maintained at that temperature for 3 h. After cooling, the activated carbon mass was washed with distilled water till washing solution attained a pH value of 6.0 then the washed material was dried at 110 C for 24 h and stored in clean dry bottles [9]. 3. Characterization methods 3.1. Determination of % ash contents, weight loss on drying, pH of the supernatant and pHpzc The ash content percentage fixed carbons were carried out on each sample of the activated carbon and the precursor. For each sample a crucible was placed in furnace at 650 C for 1 h, cooled down in a desiccator, and the weight of ignited crucible was recorded, 2.0 g sample of activated carbon was placed in a crucible and transferred into a muffle furnace at 650 C for six hours. After heating, the crucible was allowed to cool to room temperature in desiccator. The percentage ash content was determined using Eq. (1) [10, 11]. Where, Wf is the weight of sample pulse crucible after firing, Wc is the weight of empty crucible and Ws is the weight of sample before firing. The weight loss during drying was determined for the date palm pits and activated sample by weighting 0.5 g of the sample and heating for 24 h in oven at 110 C until the weight of the sample became constant using Eq. (2). By-Products of Palm Trees and Their Applications Materials Research Forum LLC Materials Research Proceedings 11 (2019) 275-285 doi: https://doi.org/10.21741/9781644900178-23 277 Where, Wb and Wa is the weight of sample before and after drying respectively pH of the supernatant was determined by adding 0.5 gm of adsorbent to 25 mL of deionized water and the mixture was shaking for 48 h, the supernatant was then filtered to remove solid adsorbent and the pH of the supernatant was measured using Jenway pH-meter. pHpzc of activated carbon was also measured as 50 mL of 0.01 M NaCl solutions was put into several closed Erlenmeyer flasks. The pH in each flask was adjusted to a value between 2 and 6 by adding HCl (0.01 M) or NaOH (0.01 M) solutions. We take 0.15 g from the sample and added them to each flask, the flasks were agitated for 48 h, and the final pH was then measured. The pHpzc is the point where pH final pH initial = zero [12]. 3.2. Scanning Electron Microscopy (SEM) analysis The surface morphology was obtained using Scanning Electron Microscope (Quanta 250 FEG) working at a high voltage of 15 kV. The samples were coated with gold by a gold sputtering device for clear visibility and conductivity. Thereafter, the coated sample was placed on the sample holder for analysis [13, 14]. 3.3.Surface area and pore size analysis The surface area and average pore size of samples were determined using a gas sorption analyzer. The adsorption-desorption isotherm of nitrogen was determined at its boiling point 196 C by ASAP 2020 instrument. The samples were degassed under vacuum at 350 C for 3h prior to measurement. The nitrogen adsorption-desorption data were recorded at liquid nitrogen temperature of -196 C. The adsorption equilibrium time was set at 60 S. The Brunauer, Emmett and Teller (BET) method was used to calculate the surface area, using the data obtained from the N2 adsorption isotherm within the P/P range of 0.05 0.30, where P is the equilibrium pressure and P is the saturation pressure. 3.4. Adsorption equilibrium studies Batch adsorption experiments were undertaken in a series of Erlenmeyer flasks containing 0.25g of ACS and equal volumes of methylene blue solutions at varying concentration (50-500 mg /L). The flasks were shaken in a thermostatic shaker at the desired temperature with shaking speed of 150 rpm for 24 h. The equilibrium concentrations of methylene blue in the supernatant were analyzed spectrophotometrically using (HPST uv/vis spectrophotometer) at λmax 662 nm. The effects contact time were studied with respectto the adsorption of MB byCP212, CP214 and CP124.The effect of temperature was studied by conducting a batch adsorption experiment at 25, 35, and 45°Cfor CP124 as a selected sample. Results and Discussion 1. Characterization of precursor and activated carbon samples. Table 1: Ash content weight loss drying, pH of supernatant and pHpzc of samples. pHpzc pH of supernatant Weight loss drying (%) Percentage ash (%) sample 6.32 5.95 13.24 0.77 DPP 3.20 2.35 4.59 5.170 CP212 3.5 2.65 4.960 4.765 CP214 4.3 3.00 5.155 3.214 CP124 Table 1 shows that ash content for DPP is 0.77%it is very low due to the higher amount of cellulose and lignin [15], while for modified AC it is in the range of 3.214–5.170 %. We can say that activating agent has the ability to dissolve apart of ash content; Moisture content is one of By-Products of Palm Trees