New adsorbents from maize cob wastes and anaerobic digestate for HS removal from biogas.

Two activated carbons (ACs) were prepared by physical activation of Maize Cob Waste (MCW) with CO, during 2 and 3 h (MCW(PA)2h and MCW(PA)3h, respectively). Two other ACs were prepared by chemical activation: a) MCW(LD) - MCW was impregnated with anaerobic liquid digestate (LD) and carbonized under N atmosphere; and b) CAR-MCW(LD) - previously carbonized MCW was impregnated with LD and carbonized under N atmosphere. All ACs were fully characterized for textural and chemical properties, and then used in dynamic HS removal assays from real biogas samples. Regarding HS removal, the ACs that were physically activated behaved much better than the impregnated ones: MCW(PA)3h and MCW(PA)2h showed HS uptake capacities of 15.5 and 0.65 mg g, respectively, while MCW(LD) and CAR-MCW(LD) showed values of 0.47 and 0.25 mg g, respectively. This may indicate that textural properties (surface area and microporosity) are more important than mineral content in HS removal. Effectively, both surface area and micropore volume were much higher for the samples of MCW(PA)3h (S = 820 m g and V = 0.32 cm g) and MCW(PA)2h (S = 630 m g and V = 0.21 cm g) than for the ACs that were chemically activated (S = 38.0 m g and V = 0.01 cm g for MCW(LD); S = 8.0 m g and V = 0.01 cm g for CAR-MCW(LD)). High oxygen content in MCW(PA)3h favoured the catalytic oxidation reaction of HS, promoting its removal. The use of MCW as precursor and LD as activating agent of the ACs may contribute for the integrated management of maize wastes and to diversify the applications of anaerobic digestate.