Modified Tea Waste: A ‘Greener’ Wastewater Treatment Technology

1 Department of Chemical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh

2 Department of Chemistry, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh

‡ indicates presenting author

* Corresponding author email: nafisaislam@che.buet.ac.bd

Abstract:

The annual tea production of Bangladesh in 2020 was more than 79 million kg,¹ with 90% of the tea left behind as waste post-extraction and consumption.² Agricultural solid wastes such as those from the tea industry can be a source of readily available, environmentally friendly sorbents for wastewater treatment.³ The adsorption efficacy of pollutants, such as methylene blue dye can be further improved by applying chemical activation agents upon tea wastes.⁴ This study evaluates the dye removal efficacies of chemically modified tea waste adsorbent, using ‘greener’ chemicals for modification. The dye removal efficacy of five types of tea waste adsorbents were studied; unmodified tea waste (U), unmodified tea waste treated with sodium hydroxide (UNA), tea waste modified with three ‘green’ acids, i.e. citric acid (M-CA), oxalic acid (M-OA) and tetraacetic acid (M-EDTA). In the case of M-CA, M-OA and M-EDTA, the modified tea waste adsorbents were stabilised with sodium hydroxide (NaOH).Equilibrium and kinetic absorption studies were carried out for all five tea waste adsorbents using UV-VIS spectroscopy for methylene blue dye concentration measurement. The tea waste adsorbents showed a range of adsorption capacities (reported in units of mg dye adsorbed per g of tea waste adsorbent used) with 732.32 mg/g, 733.53 mg/g, 1002.89 mg/g adsorbent, 670 mg/g 914.742 mg/g for U, UNA, M-CA, M-OA and M-EDTA, respectively. The dye adsorption data for all the adsorbents showed Langmuir adsorption characteristics and followed pseudo-second order kinetics. The adsorbents were further characterised using Fourier Transform Infra Red Spectroscopy. The incorporation of acidic (carboxyl) groups by esterification of the unmodified tea waste, using citric acid, oxalic acid and the incorporation of complex chelating functionalities by EDTA all contributed to the increase of dye adsorption capacity of the tea waste. The stabilizing agent, NaOH did not have effect on the performance of the raw or unmodified tea waste (data not shown). The results of the chemical kinetic analysis indicate that the chemical modification accelerated the intraparticle diffusion rate. The maximum dye removal capacity using ‘green’ chemical activators for tea waste was higher than those found in literature, and thus these novel adsorbents can be potential low-cost ‘green’ solutions for wastewater treatment.

¹ BTB: Bangladesh tea board. Annual Statistical Report; 2020.

² https://doi.org/10.1016/j.cej.2011.04.017

³ https://doi.org/10.1007/s11270-018-3869

⁴ https://doi.org/10.1016/j.cej.2011.09.100