Removal of added nitrate in cotton burr compost, mulch compost, and peat: Mechanisms and potential use for groundwater nitrate remediation

We conducted batch tests on the nature of removal of added nitrate in cotton burr compost, mulch compost, and sphagnum peat that may be potentially used in a permeable reactive barrier (PRB) for groundwater nitrate remediation. A rigorous steam autoclaving protocol (121 °C for 2 h each day for three consecutive days) for the cotton burr compost and autoclaving of all labware and the nitrate working solutions resulted in drastically different results compared to the non-autoclaved treatment. In the non-autoclaved cotton burr compost, added nitrate at 20 mg N l−1 decreased rapidly and was not detected after 3 d; whereas, the autoclaved cotton burr compost showed persistent nitrate above 15.5 mg N l−1 even after 10 d, which is comparable with nitrate concentrations above 17.6 mg N l−1 in a treatment using NaN3 at 1000 mg l−1. Dewaxed cotton burr compost showed decreased nitrate reduction compared to the pristine cotton burr compost. No nitrate reduction was detected in the dewaxed sphagnum peat. It is concluded that nitrate removal in the organic media is controlled by microbiologically mediated processes. The use of readily available cotton burr and mulch composts may offer a cost-effective method of nitrate removal from contaminated groundwater.