From Fruit Waste to Water Purifier: Plum Stone-Derived Biochar for Malachite Green Removal

Agriculture is a crucial sector of Serbia's economy, with approximately 5 million hectares of agricultural land, accounting for about 55% of the country's territory. The fruit production/processing industry plays a significant role in economy revenue. The annual fruit production accounts around 1.5 million tons, where the plum production is approximately 380,000 t/y, positioning Serbia as one of the world's leading producers of plums. Beside the pruning waste, the plum production and processing result in nearly 11,000 tons of plum stones (PLS). Currently, there is no structured system for the collection or repurposing of this byproduct, leading to its disposal in landfills or through incineration. In this paper, PLS, a renewable, low-cost, and sustainable byproduct of the fruit processing industry, was utilized to produce biochar (PLS-B) through thermochemical conversion (pyrolysis). The resulting biochar was characterized through Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), and zeta potential analysis. It was subsequently used as an adsorbent for the removal of the carcinogenic dye Malachite Green (MG) from water solution. Results demonstrated that PLS-B possesses a highly developed porous structure and a negatively charged surface, which facilitates the adsorption of cationic molecules such as MG. The adsorption isotherm of MG on PLS-B was determined by mixing 0.1 g of PLS-B with 50 mL of MG solution (10–350 mg/L) at room temperature for 24 h (batch system). The experimental data were fitted with Langmuir, Freundlich, and Sips isotherm models. The Sips isotherm model provided the best fit, with a maximum adsorption capacity of 99.89 mg/g. The utilization of fruit industry waste to produce an efficient adsorbent offers a potential solution to disposal challenges, reduces environmental pollution on local, regional, and global scales, and simultaneously purifies wastewater contaminated with toxic dyes.