NH4OH-modified corncob biochar as a potential agent for improving soil properties, immobilizing organic and inorganic pollutants

The increasing reliance on chemically intensive agriculture has led to serious environmental issues, such as the problem of agricultural waste, soil degradation, and water contamination. Also, the widespread use of tetracycline and silver-based products has caused persistent contamination that threatens the environment and human health. Additionally, large amounts of agricultural waste, such as corncobs, are generated annually, i.e., in 2023, over 1.17 billion metric tons of waste were generated, and they are often underutilized. Converting waste into biochar (BC), a material with potential uses in soil regeneration, pollutant removal, and carbon sequestration, offers a sustainable solution to all these issues (Zheng et al., 2023). However, sometimes the physicochemical properties of this material may be unsuitable for its intended use, and modification may be necessary (An et al., 2023). This study explores NH₄OH-modified corncob biochar as an eco-friendly, low-cost adsorbent for the removal of silver ions (Ag⁺), silver nanoparticles (AgNPs), and tetracycline (TC), aiming to improve agricultural waste management and protect the environment.

The biochar (BC) was produced from corncob (C) at 700 °C for 1 h, with a heating rate of 12 °C/min. The 25% NH₄OH solution was used to chemically alter the biochar surfaces. 20 grams of biochar were introduced in a 1:5 (s/v) ratio to 100 ml of NH₄OH in a magnetic stirrer and then, the mixture was stirred at room temperature for 2 h. The excess ammonium hydroxide was then eliminated using a paper filter. The resulting biochar (BCM) mixture was heated to 300°C for around 2 h in a furnace after being allowed to air dry overnight. Adsorption of the pollutants was conducted in batch experiment, with an initial concentration of 100 mg/L for Ag⁺, 100 mg/L for AgNPs, and 10 mg/L for tetracycline at a pH of 6. The concentration of Ag⁺ was measured using a silver-ion-selective electrode, while that of AgNPs, using a UV-Vis spectrophotometer at 437 nm. The concentration of tetracycline was determined using HPLC.

The performed modification changed the surface functional groups of BC. In the FTIR spectra, the introduction of amine groups was observed. The basic functional group increased by 31.8%. In addition, the hydrophobicity of the solid was reduced by 23.68%. BCM had point of zero charge (pH_PZC) of 9.98, specific surface area (S_BET) of 144 m²/g, whilst the content of basic and acidic functional groups equalled 8.13 mmol/g and 4.75 mmol/g, respectively. Adsorption of all adsorbates reduces in mg/g as the dose of the adsorbent increases from 0.01 to 0.1 g. There was an insignificant difference in the absorbed amounts of Ag⁺ and AgNPs at pH 3, 6, and 9, whilst TC demonstrated a significant difference, with the highest adsorption recorded at pH 9. TC and AgNPs demonstrated competitive interaction, while TC and Ag⁺ showed synergistic interaction in the bi-sorbate system. Modified biochar offers an excellent efficiency in the removal of most pollutants from the environment. Its highest observed adsorption capacity was towards AgNPs and equalled 26.73 mg/g.

 

Keywords: Antibiotics, contaminant adsorption, crop residue, agrochemicals, modified biochar