A comparative study of adsorbents derived from coal fly ash for the adsorption of tetracycline from aqueous solutions

Abstract

Emerging contaminants (ECs) like tetracycline (TC) are generated from various human activities, such as pharmaceutical manufacturing, agricultural runoff, and wastewater discharge. The presence of TC poses severe risks to the environment and public health. Various methods have been proposed for removing TC from water, such as chemical precipitation, membrane filtration, and advanced oxidation processes. However, adsorption has gained prominence as an effective method for removing tetracycline due to its simplicity, cost-effectiveness, and ability to achieve high removal efficiencies. Fly ash is a hazardous byproduct of coal combustion, often discarded in landfills, posing environmental risks. However, it can be repurposed into low-cost adsorbents through various modifications and treatments. Utilizing fly ash-derived adsorbents for tetracycline removal not only helps reduce waste but also provides an eco-friendly and cost-effective alternative to conventional, more expensive adsorbents. This study aimed to develop and evaluate various fly ash (FA)-derived adsorbents for TC removal, leveraging waste materials for environmental sustainability.

Firstly, FA was acid-treated with hydrochloric acid to produce acid-modified FA (AM-FA). Secondly, FA was base-treated with sodium hydroxide to yield base-modified FA (BM-FA). Additionally, Zeolite Na-P1 (ZNa-P1) was synthesized from FA using hydrothermal treatment. These three adsorbents were subjected to adsorption tests to compare their adsorption performance. Furthermore, silica nanoparticles (SiNPs) were derived from FA (FA-SiNPs) and subsequently FA-SiNPs was doped with iron to create Fe-SiNPs. For comparison, silica nanoparticles were also synthesized directly from a pure sodium silicate solution (SSSNPs). All silica nanoparticle-based adsorbents (i.e. FA-SiNPs, Fe- SiNPs, SSSNPs) underwent adsorption tests to compare their adsorption efficiency.

The comparative adsorption test among FA, AM-FA, BM-FA and ZNa-P1 revealed that BM-FA and ZNa-P1 removed 76 % and 90 % of TC, respectively, compared to 35 % with unmodified FA. AM-FA had the lowest performance, removing just 11 % of TC. ZNa-P1's superior performance was linked to its high zeolite purity, with a high cation exchange capacity (CEC) of 6.37 meq/g and a surface area of 35.7 m2/g. BM-FA, had a larger surface area of 110.8 m2/g, but exhibited a lower CEC of 3.42 meq/g. The adsorption efficiency of these adsorbents was more closely related to CEC than surface area. Optimal TC removal with ZNa-P1 was achieved at 7.5 g/L dosage and pH 5. The adsorption of TC on ZNa-P1 followed pseudo-second-order kinetics and the Langmuir isotherm model, with a maximum capacity of 46.34 mg/g at 30 °C. Thermodynamic studies with ZNa-P1 indicated that the process was spontaneous and endothermic. The adsorption mechanism of TC on ZNa-P1 involved ion-exchange, hydrogen bonding, and electrostatic attraction.

The comparative adsorption tests among FA-SiNPs, SSSNPs, and Fe-SiNPs revealed that Fe-SiNPs demonstrated superior performance, removing 59 % of tetracycline, compared to 30 % and 20 % removal by FA-SiNPs and SSSNPs, respectively. The enhanced removal efficiency of Fe-SiNPs was attributed to the iron content, which facilitated TC adsorption through chelation. Optimal TC removal using Fe-SiNPs was achieved at a dosage of 5 g/L and within a pH range of 4-5. The adsorption of TC on Fe-SiNPs followed Elovich kinetics and the Langmuir isotherm model, with a maximum capacity of 32.31 mg/g at 30 °C. Thermodynamic studies with Fe-SiNPs indicated that the adsorption process was spontaneous and exothermic. The adsorption mechanism of TC on Fe-SiNPs was chemisorption involving electrostatic attraction and hydrogen bonding.

This study highlights the potential of FA-derived adsorbents, particularly ZNa-P1 and Fe-SiNPs, as sustainable solutions for removing TC from contaminated water. The findings contribute to advancing waste utilization strategies particularly, adsorption, for environmental remediation.