Ni-magnetite nanoparticle as additive in cell immobilization in biohydrogen production

Previously, the cell immobilization technique using granular activated carbon was implemented due to its porous nature and large surface area, which promote microbial activity and attachment. However, low hydrogen productivity was still widely reported. So, this study aims to utilize nickel (Ni) and magnetite (Fe3O4) as additives in granular activated carbon (GAC), namely, GAC-NiFe3O4, to study its ability as microbial support carriers in thermophilic biohydrogen production. From the study's first objective, this work has successfully synthesized GAC-NiFe3O4 originated from coconut shells embedded with the additives using the co-precipitation method. GAC-NiFe3O4 demonstrated a higher hydrogen production rate (HPR) of 65.49% than control (GAC). The analysis of variance showed a reliable optimization model with an R-squared of 0.93 and the predicted HPR of 21.21 mL H2/L.h. The result was further validated at 20.33 ± 0.32 mL H2/L.h with a 4.16% error from the predicted value. For the second objective, GAC-NiFe3O4 was successfully immobilized with mixed culture in an attached biohydrogen production system. The immobilization utilizing GAC-NiFe3O4 shortened biohydrogen production from 66-d to 26-d, incrementing hydrogen yield (HY) by 57.30%. Subsequently, the effect of sludge on the GAC-NiFe3O4 ratio, which ranged from 1:0.5-4, was studied. The optimum HY of 1.64 ± 0.04 mol H2/mol sugar consumed was achieved at a ratio of 1:1. In addition, the optimum initial sugar (ranges 10-30 g/L) concluded from the MONOD model was at 20 g/L, with maximum specific growth rate, specific growth rate, and cell growth saturation coefficient of 2.05 h-1, 1.98 h-1, and 6.96 g/L, respectively. The sequencing analysis revealed that the dominant species was Thermoanaerobacterium sp. at 99% abundance. In the third objective, this work has successfully optimized operational parameters in terms of pH (6-7), temperature (60-80°C), and agitation (60-180 rpm) utilizing the optimum immobilized GAC-NiFe3O4 obtained previously. The optimal pH obtained was at pH 6.0 with HY of 2.66 ± 0.09 mol H2/mol sugar consumed. At 60°C, the highest HY was obtained at 2.75 ± 0.11 mol H2/mol sugar consumed. The optimal agitation obtained was at 120 rpm, attaining the highest HY of 3.44 ± 0.54 mol H2/mol sugar consumed. Furthermore, in the solid loading rate (SLR) study, which ranges from 5-25%, 10% of SLR was found to be optimum, with 66.67% total suspended solids degradation. The optimum hydraulic retention time was 3 h from 3-48 h with an energy production rate of 29.19 kJ/L. Following all the previous optimum conditions obtained, the final objectives were successfully achieved. This work has characterized the collected POME in terms of physiochemical, heavy metal, and sugar analysis. The total carbohydrates (TC) study ranges from 10-100%, showing 50% of POME POME (TC= 14.64 ± 0.11 g/L) can be used to obtain maximum HY with 94.12% sugar utilization. Thermoanaerobacterium sp. was identified as the dominant species at 67% abundance. GAC-NiFe3O4 showed potential for heavy metal removal in POME from 29.71-99.86%. Overall, applying GAC-NiFe3O4 showed high potential in biohydrogen production while providing a sustainable alternative to the bioremediation of POME.

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