This research hypothesizes that agricultural residues can be transformed into value-added adsorbents with the ability to remove arsenic from water effectively. Modifying these residues is expected to enhance adsorption capacity, providing a promising solution to address the global issue of arsenic contamination in water.

Arsenic (As) contamination in drinking water poses a significant global water health hazard. To address this issue, adsorption has gained increasing attention as a promising technology for As removal, offering improved cost efficiency and simplified treatment operations. The utilization of agricultural biomass residues as As adsorbents has emerged as a promising approach. However, untreated biomasses exhibit limitations such as low sorption capacity, coloration issues in treated waters, increased BOD, COD, and TOC levels due to the release of soluble organic compounds, poor solubility and stability, and their unsuitability for conventional process equipment due to their fragile nature and swelling. It is evident that modification of biomasses is necessary to overcome these drawbacks, enabling their application at commercial and industrial scales.

Metal oxides, such as iron oxide, have demonstrated high efficacy as As adsorbents. However, due to their small particle size, using them in adsorption systems presents challenges. In the last two decades, the use of the terms engineered- or modified-biomass/biochar has been gaining momentum in the literature. These terms refer to chemically treated biomasses/biochars produced to improve adsorption capacities. Generally, the modification process consists of impregnating the biomass/biochar with a metal agent with a high affinity to the target contaminant (e.g., As). The resulting composite material consists of the original biomass or biochar acting as a support structure for the metal oxide, which serves as the active site for adsorption. This modification process can significantly improve the adsorption capacity, selectivity, and stability of the material, making it a more effective adsorbent for water treatment applications.

The modification process is typically conducted by mixing the untreated biomass/biochar materials in a chemical solution. However, this process can be time-consuming, is difficult to control product formation, and results in potentially unstable and unreproducible physicochemical properties of the modified materials. Clearly, it would be beneficial to have a better method for this modification process.

The electrochemical modification offers a promising and simplified approach to overcome the limitations associated with chemical-based methods for preparing modified biomass/biochar adsorbents. This technique involves a two-electrode system within an electrochemical cell, where a sacrificial anode (such as Fe) eliminates the need for external chemical modifiers. The modification process begins by passing an electric current through the electrodes in an electrolyte solution, while the biomass/biochar materials are continuously stirred within the solution between the electrodes. As the sacrificial anode dissolves, the modifying agent ions (e.g., Fe²⁺) are released and undergo a series of reactions, resulting in the deposition of the desired modifying agent (e.g., iron oxide) onto the surface of the biomass/biochar.

The overall objective of the conducted research over three years was the development of biomass/biochar-based adsorbents with high As adsorption capacities using chemical and electrochemical modification methods. Seven different adsorbents, including chemically iron-loaded biomass (ICS), electrochemically iron-loaded biomass and biochar (OBM and OBC), microwave-assisted and electrochemically iron and manganese-loaded biochar (MnBC and FeMnBC), and microwave-assisted and electrochemically aluminum and zinc-loaded biochar (ZnBC, and AlZnBC) were developed and used for As adsorption. The effects of parameters affecting the As adsorption capacity of the modified adsorbents were investigated, and the modification conditions were optimized. The concept of the shrinking core model was applied to develop a mathematical model to characterize the As adsorption process for OBM and OBC. A diffusional mass transfer model (DMTM) was developed to identify the external mass transfer coefficient (k_f), effective pore volume diffusion coefficient (D_p), and surface diffusion coefficient (D_s) in the adsorption of As onto MnBC and FeMnBC. Finally, a model was developed to study temperature distribution inside the biomass during pyrolysis for making ZnBC.

Overall, the modified adsorbents prepared through the applied modification methods exhibited exceptional performance in removing both As(III) and As(V) from water. However, their adsorption capacities varied depending on the optimization considerations for the modification conditions, resulting in higher efficiency for either As(III) or As(V) removal. The combination of microwave pyrolysis and electrochemical treatment demonstrated significant potential as an alternative to traditional pyrolysis and chemical treatment processes, showcasing its viability for preparing biomass/biochar-based adsorbents.

Moreover, extensive investigations were conducted to assess the influence of different adsorption conditions on the As adsorption capacity of the modified adsorbents. Parameters such as pH, temperature, initial As concentration, and adsorption time were systematically studied, revealing their significant impact on the adsorption capacity. By analyzing the effects of these adsorption conditions, valuable insights were obtained regarding the underlying mechanisms governing the adsorption process. Notably, chemisorption was found to be the dominant and rate-limiting mechanism in most cases. This understanding of the adsorption mechanisms is crucial for optimizing the adsorption process and designing effective As removal strategies using the modified adsorbents.

This research provides a substantial contribution to the field by offering valuable insights into the conversion of agricultural residues into value-added and efficient adsorbents for As removal. By exploring different modification methods and optimizing the adsorption conditions, this study paves the way for the development of enhanced water treatment approaches to address the challenges posed by As contamination. Moreover, the findings have the potential to be extended to the development of adsorbents for other contaminants. By harnessing the potential of biomass-based adsorbents, this research not only expands our knowledge regarding the utilization of agricultural residues but also provides practical solutions for mitigating water pollution issues.