The research team - Pham Trung The¹, Huynh Thi Ngoc Han², Vu Ngoc Toan³, Nguyen Huynh Minh Duy⁴, and Ho Thi Thanh Van*¹ — successfully produced activated carbon (AC) from coffee grounds, achieving up to 92% tetracycline removal from aqueous solutions.

Affiliations:

¹ Interdisciplinary Science Institute, Nguyen Tat Thanh University

² Faculty of Environment, Ho Chi Minh City University of Natural Resources and Environment

³ Institute of New Technology, Hanoi

⁴ Institute of Applied Research and Green Technology Transfer

Tetracycline: A Persistent Environmental Risk

Tetracycline (TC) is a broad-spectrum antibiotic widely used in medicine, livestock, and aquaculture. However, TC is poorly biodegradable and can persist in the environment. When released into water systems, it can: (1) promote the development of antibiotic resistance genes, (2) increase bacterial resistance, and (3) cause ecological toxicity that affects aquatic organisms and human health. Even at concentrations as low as 1 ng/L, the presence of TC is considered hazardous.

Conventional treatment methods — such as biological degradation, chemical oxidation, and photocatalysis — are often complex or costly. By contrast, adsorption has emerged as a simple, effective, and economical technique, particularly when using activated carbon.

From coffee waste to functional material

Coffee grounds, an abundant by-product in Vietnam, are often discarded, leading to waste and potential environmental concerns. The research team selected coffee grounds as a precursor to produce activated carbon via a two-step process:

1.    Hydrothermal carbonization (HTC): performed at 240 °C for 5 hours to convert wet coffee grounds (~80% moisture) into hydrochar (HC).

2.    Chemical activation: treatment with KOH at 650 °C under nitrogen atmosphere, enhancing pore structure and increasing specific surface area.

The results were significant:

-    Specific surface area increased from 23.06 m²/g (HC) to 976.34 m²/g (AC).

-    Pore volume increased from 0.047 cm³/g to 0.535 cm³/g.

-    Micropore size of ~2.19 nm facilitated adsorption of TC molecules.

Scanning electron microscopy (SEM) images showed that AC had a highly porous structure compared to the dense surface of HC. Fourier-transform infrared spectroscopy (FT-IR) confirmed the reduction of oxygen-containing functional groups, improving interaction between AC surfaces and TC molecules.

Adsorption performance

Batch adsorption experiments were carried out under varying pH, AC dosage, initial TC concentration, and contact time. The key findings were:

-    Optimal pH: 3, with removal efficiency above 85%, reaching nearly 92%.

-    Optimal dosage: 1.5 g/L of AC.

-    Adsorption equilibrium achieved in 100 minutes.

As the initial TC concentration increased from 10 ppm to 50 ppm, adsorption capacity rose from 13.68 mg/g to 64.27 mg/g.

The researchers noted that at low TC concentrations, the interaction between pollutants and AC surface was limited; at excessively high AC dosages, adsorption equilibrium was reached too quickly, making kinetic evaluation less accurate.

Implications and future prospects

This study demonstrates that spent coffee grounds are a promising, low-cost precursor for producing high-performance activated carbon. The material not only effectively removes tetracycline but also contributes to reducing agricultural waste and advancing circular economy practices. Moreover, it holds potential for application in the removal of other antibiotics and organic pollutants.

The research team intends to extend their work to other contaminants and to further investigate adsorption kinetics and isotherm models in order to optimize the process./.