Abstract
In this study, we evaluated the use of prickly pear seed cake, a by-product of prickly pear seed oil extraction, as a new precursor for producing activated carbon by phosphoric acid activation, and the obtained carbon’s capacity for heavy metal removal from aqueous solution. Response surface methodology based on the full factorial design at two levels (24) was developed to reduce the number of experiments and reach optimal preparation conditions for the removal of cadmium and lead ions from aqueous solutions. Design Expert 11.1.2.0 Trial software was used for generating the statistical experimental design and analyzing the observed data. Factors influencing the activation process, such as carbonization temperature, activation temperature, activation time, and impregnation ratio, were studied. Responses were studied in depth with an analysis of variance to estimate their significance. Each response was outlined by a first-order regression equation demonstrating satisfactory correspondence between the predicted and experimental results as the adjusted coefficients of correlation. Based on the statistical data, the best conditions for the removal of heavy metals from aqueous solution by the obtained activated carbon were indicated. The maximum iodine number and methylene blue index were 2527.3 mg g−1 and 396.5 mg g−1, respectively, using activated carbon obtained at the following conditions: Tc = 500 °C, Ta = 500 °C, impregnation ratio = 2:1 (g H3PO4: g carbon), and activation time of two hours. The maximum adsorption reached 170.2 mg g−1 and 158.4 mg g−1 for Cd2+ and Pb2+, respectively, using activated carbon obtained at the following conditions: Tc = 600 °C, Ta = 400 °C, impregnation ratio = 2:1 (g H3PO4: g carbon), and activation time of one hour. The activated carbon obtained was characterized by Boehm titration, pH of point of zero charge (pHPZC), Brunauer–Emmett–Teller surface area (SBET), and scanning electron microscopy. Adsorption was performed according to different parameters: pH solution, adsorbent dosage, temperature, contact time, and initial concentration. Regeneration experiments proved that the obtained activated carbon still had a high removal capacity for Cd2+ and Pb2+ after five regeneration cycles.
Highlights
Motivated by the search for a new precursor, prickly pear seed cake was considered in this study
Pear seed cake has a significant proportion of lignin of 19.2% (Table 1), which is comparable to Pituranthos chloranthus (17.6%) and Retama raetam (20.5%) [47], and still lower than that observed for tamarix aphylla (30.0%) [44]
We demonstrated that activated carbon synthesized from prickly pear seed cake is suitable for heavy metal removal from industrial wastewater
Summary
Toxic and carcinogenic heavy metals include cadmium (Cd) and lead (Pb). Their main sources are the wastewater treatment stations of various industries [8]. Several treatment technologies have been adopted for the removal of heavy metals from wastewater, such as electrochemical treatment [12], reverse osmosis [13,14], flocculation [15], precipitation [16,17], membrane filtration [18], ion exchange [19,20], and adsorption [21,22]. The fruit consists of about 10–15% seeds [35] These seeds are usually rejected as waste, their use as a by-product generates a novel and valuable source of oil [36], as well as functional ingredients that are used in the food industry (seed cake). Preparing activated carbons with prickly pear seed cake is an interesting approach to valorize this raw material [37,38]
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