Sequential removal of nitrate and sulfate in woodchip and hematite – coated biochar bioreactor

Abstract

Laboratory column experiments have been used to study the sequential removal of nitrate (NO₃⁻) and sulfate (SO₄²⁻) from mine water, where NO₃⁻ was removed through denitrification and SO₄²⁻ was removed through SO₄²⁻ reduction and the subsequent precipitation of hydrogen sulfide (H₂S) in a hematite-coated biochar (HCB) bioreactor. Denitrification and SO₄²⁻ reduction were investigated in columns filled with pine woodchips and pine woodchips + biochar, both with and without the addition of lactate. Experimental results indicated that a >90% NO₃⁻ removal from 50 mg L⁻¹ NO₃⁻-N was achieved at a hydraulic residence time of 5 days without lactate addition, but that SO₄²⁻ reduction was minimal after an initial startup period. Lactate was added to stimulate SO₄²⁻ reduction, producing H₂S with >90% SO₄²⁻ removal from an initial concentration of 361 mg L⁻¹ SO₄²⁻-S. Sulfate concentrations were reduced to a greater extent in the woodchip + biochar column, and NH₄⁺ production was enhanced in both columns after lactate addition. After treatment in the HCB columns, H₂S and NH₄⁺ were removed to >95%. X-ray photoelectron spectroscopy (XPS) indicated that S²⁻, S₂²⁻, S⁰ and NH₄⁺ were accumulating in the HCB columns and surface-bound iron was converted from Fe(III) to Fe(II). The XPS results suggested that the reductive dissolution of hematite preceded the precipitation of H₂S as FeS, pyrite and elemental sulfur on the HCB surfaces.