Rates of Sulfate Reduction Achieved in Columns Based on Untreated Sugarcane Bagasse for Metals Removal

Groundwater contamination by mining effluents and spills at industrial sites often involves toxic metals and sulfate. Passive remediation systems, such as permeable reactive barriers, are attractive alternatives to traditional pump-and-treat. The rates of bacterial sulfate reduction for metals precipitation were assessed in three biocolumns assembled using sugarcane bagasse in natural form as the carbon and energy substrate for sulfate-reducing bacteria. An abiotic experiment receiving glutaraldehyde was assembled for comparison. An input solution containing 5,000–$\mathrm{6,004}\mathrm{mg}/\mathrm{L}$ sulfate, 10.0–$15.0\mathrm{mg}/\mathrm{L}$ zinc, and 10.0–$15.2\mathrm{mg}/\mathrm{L}$ nickel was supplied to the columns at $126.7\mathrm{mL}/\mathrm{day}$ for 145 days or 19.3 pore volumes of flow. From 30 to 145 days, pH increased from 5.5–5.8 to 6.8–8.0, the oxidation-reduction potential (ORP) declined from positive values to approximately $-400\mathrm{mV}$, sulfate concentrations declined by approximately $\mathrm{1,000}\mathrm{mg}/\mathrm{L}$, and metals (Zn and Ni) declined to nondetectable levels. Scanning electron microscopy with energy dispersive spectroscopy (EDS) confirmed Zn and Ni with S in the solid phase. The achieved rates of sulfate reduction spanned from 0.27 to 0.92 mg ${\mathrm{SO}}_4^{2-}/\mathrm{L}$ per day per initial gram (dry basis) of sugarcane bagasse, surpassing the maximum rate reported in the literature for a leaf-mulch, wood-chip, sawdust column. In contrast, the abiotic-column ORP remained at $+150\mathrm{mV}$, pH remained in the 5.5–6.5 range, and sulfate and phosphate underwent nonreactive transport. Also, nickel was not removed. On the contrary, measured Ni release was greater than the input concentration. For zinc, the data suggested adsorption onto bagasse, with breakthrough retardation. Sugarcane bagasse appears a suitable substrate for passive in situ groundwater cleanup in many areas of the world where this residue abounds.