Production and characterization of H₂S and PO₄³⁻ carbonaceous adsorbents from anaerobic digested fibers

Anaerobic digestion (AD) is an important technology to produce biogas from dairy manure. Although the AD of dairy manure results in the harnessing of the energy contained in manure, most of the nutrients (phosphorous and nitrogen) remain in the liquid effluent, representing an important source of pollution. Additionally, the biogas produced contains H₂S and CO₂, limiting its practical use as fuel. In this paper, we report the production and use of a carbonaceous adsorbent from AD fibers for the removal of hydrogen sulfide (H₂S) from biogas and phosphate (PO₄³⁻) from aqueous liquid effluents. The adsorbents studied were produced via slow pyrolysis between 350 and 800 °C followed by CO₂ activation. The elemental and proximate analyses, surface area and pore size distribution of each of the adsorbents studied are reported. Their adsorption capacities were assessed using H₂S breakthrough and PO₄³⁻ batch equilibrium tests. The sorption capacity varied between 21.9 and 51.2 mg g⁻¹ for H₂S and between 4.9 mg g⁻¹ and 37.4 mg g⁻¹ for PO₄³⁻. Commercially available activated carbon studied adsorbed 23.1 mg g⁻¹ H₂S and 15.7 mg g⁻¹ PO₄³⁻. The results show that the retention of H₂S and PO₄³⁻ compounds were governed by the ash content, surface area and pH. Adsorption mechanisms for H₂S and PO₄³⁻ sorption are proposed.