Production of granular activated carbon by thermal air oxidation of biomass charcoal/biochar for water treatment in rural communities: A mechanistic investigation

Abstract

Small communities in remote areas of the United States often lack adequate water treatment services. Adsorption by granular activated carbon (GAC) is a vital drinking-water purification approach in small water treatment plants and point-of-entry and point-of-use systems. This study shows that rural communities can utilize a simple approach, thermal air oxidation (TAO), to produce their own GAC from locally available biomass materials. Unlike commercial GAC production processes that require high temperatures and activating gasses, activation of charcoals (biochars) by TAO can be achieved at moderate temperatures of 400‒500 °C without a regulated supply of activating gasses. After a brief TAO for 30‒40 min, charcoals/biochars showed significant increases in the surface area by up to 80-fold, oxygen-containing functional groups, porosity, and adsorption of test organic contaminants, including one anionic herbicide, two neutral triazine herbicides, and one natural estrogen. The GAC produced by the TAO process had a N2 B.E.T. surface area of up to approximately 850 m2/g, which is comparable to that of commercial GAC. The pore size distribution (PSD) of TAO-generated GAC was narrow and peaked at an approximate pore width of 0.8‒0.9 nm. Both the surface area and PSD were significantly affected by the conditions of TAO. GAC made from walnut/almond charcoals exhibited the highest adsorptivity, despite a relatively low surface area. This simple, innovative GAC production approach is suitable for use in small community- and household-scale applications, and therefore may benefit rural communities, water treatment professionals, farmers, and the GAC industry as a whole.

Keywords

• Water treatment
• Biochar
• Oxidation
• Oxygen functionality
• Porosimetry
• Pore size distribution