| dc.description.abstract | Due to the industrialization and population growth in recent years, Vietnam and other developing countries have begun to face problems of increasing environmental pollution, including water contamination with hazardous substances. Of water and wastewater treatment methods, adsorption is considered a relatively low-cost and effective means favored by developing countries for the removal of harmful, non-biodegradable pollutants from contaminated water. Because agriculture still plays a vital role in the economy of Vietnam, it becomes clear that the abundance of agricultural wastes can be valuable feedstocks of carbonaceous sorbents used for pollution handling in Vietnam. While activated carbon (AC) derived from agricultural residues has been used as a preferential sorbent in this regard, the traditional way of AC synthesis involving processes of carbonization and activation under high-temperature (600–1200 °C) conditions makes AC an expensive, eco-unfriendly material. Hence, there is a need for the development of carbon-based adsorbents via a simpler, greener, and robust way for effective use in dealing with pollution. Recent attention has been drawn to hydrochar (HC), as this carbonaceous material is prepared through hydrothermal carbonization at low temperature (180–350 °C) and thus the richness of surface oxygenated functionality can be maintained. This study thus explores the potential of low-cost adsorbents derived from agricultural wastes in removal of typical ionic contaminants such as cationic dyes (using methylene blue, MB, as the model compound), antibiotics (tetracycline, TC, as the targeted drug), and metal species (Cu2+, Cd2+ as the tested ions) from aqueous solution.
Prior to adsorption tests, all synthetic sorbents were characterized through the SEM, SBET analyzer, FTIR, XPS techniques, and Boehm titration to determine the acidic functional groups. First, hydrochars were derived from wasted orange peels (raw-hydrochars) and further modified with nitric acid (oxidized-hydrochars) to adsorb MB. Results show that the maximum MB adsorption capacity at 30 oC estimated by the Langmuir model followed by the order of mGH (246 mg/g) > mOPH (107 mg/g) > OPH (59.6 mg/g) > GH (54.8 mg/g). Second, teak sawdust was used to synthesize ACs through hydrothermal carbonization followed by chemical activation with varying concentrations of ZnCl2 or K2CO3. For ACs, their MB-, Cd(II)-, and Cu(II)-adsorption capacity increased with the concentration of the activating agent: the maximum adsorption capacities were achieved when the weight ratio of the carbonaceous material to ZnCl2 reached 1.75. The maximum adsorption capacities obtained for MB, Cd(II), and Cu(II) were 516 mg/g, 166 mg/g, and 159 mg/g, respectively. Finally, because TC is a pH-tunable compound, it was used to validate the adsorption pathways concluded from prior tests with those higher adsorption capacity-HC and AC materials. The maximum adsorption capacities of TC estimated by the Langmuir model were found to follow the order: ACZ1175 (257.28 mg/g) > mGH (207.11 mg/g) > WAC (197.52 mg/g) > mOPH (168.50 mg/g) > OPH (85.79 mg/g) > GH (75.47 mg/g) at 25 oC and pH 5.5. In addition, potential adsorption mechanisms were deeply discussed in this study. The electrostatic force was identified as the primary pathway that led to the adsorption of the tested contaminants onto the sample. Further, while the π-π and n-π interaction became minor pathways for MB and TC adsorption onto oxidized-hydrochars, the complexation reaction was an important mechanism responsible for the adsorptive interaction between ACs and metal species (Cu2+, Cd2+). Moreover, the result illustrated that the amount of oxygen-containing functional groups is regarded as an important factor in determining the adsorptive amounts.
It is expected that the knowledge obtained through extensive exploration in this study would help further development of the low-cost materials for the practical applications. | en_US |