Treatment of Highway Stormwater Runoff Using Sustainable Biochar: A Review

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Abstract
Human activities and atmospheric deposits on highways have led to an increase of pollutants in highway stormwater runoff. The reduction of these pollutants to a minimum acceptance level has been a great subject of concern for the US Department of Transportation and US Environment Protection Agency. Researchers have investigated several methods; however, biochar utilization, an environmentally and economically favorable way for pollutant removal from highway stormwater runoff, has not been studied extensively. Biochar has been intensively used for soil improvement and water treatment. Its unique carbon structure, large surface area, and higher pore structure have made it a potential candidate for water retention and contaminant removal. This literature review focused on the removal of highway stormwater runoff pollutants such as suspended solids (dust, sludge, and leaves), heavy metals (Ni, Zn, Pb, Cr, Cd, and Cu), nutrients (nitrate and phosphate), and organic contaminants (paraffin, nitrobenzene, and p-nitrotoluene) using biomass-derived biochar. This comprehensive literature review introduces biochar production and the effect of the production process operating temperature on biochar’s structure and performance. It also analyzes the impact and efficiency of biochar on pollutant removal and illustrates its potential in the treatment of highway stormwater runoff. Highway networks impact water bodies and watersheds by collecting and transporting pollutants through stormwater runoff. These pollutants are detrimental to aquatic lives and water quality and include solids (dissolved and suspended), heavy metals (zinc, lead, copper, and so on), and nutrients (nitrogen and phosphorus, and so on). Biochar has advanced properties that allow the treatment of pollutants in water through physical, chemical, and biological adsorption processes. Biochar is generally produced using waste sources such as biosolids, wood, or farm manure. Hence, biochar is considered a cost-effective and sustainable method for highway stormwater runoff treatment. Biochar is used as performance-enhancing devices (PEDs) for best management practices (BMPs) to enable high pollutant removal performance. Biochar can be added to topsoil on highway bioslopes and filter strips to amend the topsoils and increase pollutants removal through infiltration. Biochar is incorporated as PED with other BMPs such as bioretention ponds and gardens for stormwater quality improvement. In addition, biochar is used to decrease hydraulic conductivity and increase water holding capacity of topsoil. Ultimately, topsoil application allows the use of biochar on agricultural lands for specialized crop cultivation.

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