Biochar is a highly porous media produced by pyrolyzing biomass in the absence of oxygen. Ash wood is an easily available biomass source and has shown some ability to immobilize aqueous arsenic (As); when the biomass or biochar is impregnated with ferric iron (FeIII), the material surface serves as a preferable binding site for anionic contaminations like arsenate (AsV) and arsenite (AsIII). Thus, the application of iron-impregnated biochar as an inexpensive and sustainable filter media for arsenic is widely studied because treating drinking water is logistically challenging in remote regions with widespread arsenic contamination, such as Southeast Asia. In this contribution, we explore the influence of ferric iron impregnation methods on the adsorption capacity of ash wood biochar and test the hypothesis that the amorphous iron content is a robust predictor of the capacity and/or affinity of the biochar surface for arsenic. The study evaluates equilibrium and kinetic aspects of arsenate and arsenite adsorption onto biochars produced through a variety of methods (e.g. different pH iron soaking solutions; iron:biochar ratios; ultrasonic treatment). Adsorption isotherm data are fit with Langmuir models to determine adsorption capacity and affinity. X-ray diffraction and BET analysis are then used to characterize iron mineralogy and calculate the biochar’s surface area. Flow-through column studies were integrated with a one-dimensional transport model to test isotherm-determined distribution coefficients and explore the scalability of biochar filters to household-scale filtration applications. Biochar impregnated with acidic ferric iron through ultrasonic treatment exhibited the highest iron content (70.3 mg Fe/g biochar) and adsorption capacity, with an affinity of 3.93 mg As/g biochar. Flow-through column experiments confirmed the effectiveness of the iron-modified biochar in a flow-through filter context for both well-mixed and layered biochar-sand media. Recommendations will be provided regarding optimal methods for biochar preparation as well as water treatment capacity for water contaminated with 50 to 1,000 µg/L arsenate.

Production of iron-impregnated biochar for arsenic sorbing water filters

ACS Fall 2021

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