Enhanced nitrate removal and nitrogen-selective conversion mechanism of a combined sponge iron/biochar/manganese sand system

To find a method for efficient removal and conversion of inorganic nitrogen compounds in three reduction systems, which are the sponge iron (s-Fe0),the sponge iron + biochar (sFe0/BC) micro-electrolysis system and the sponge iron + biochar + manganese sand (s-Fe0/ BC/MS)-enhanced micro-electrolysis system, the studies have been completed on the conditions of different material ratios and initial pH. When the initial pH is 7, the nitrate removals reached to 7.6%, 61.5% and 80.3%, the nitrogen ratio of products reached to 5.8%, 22.4% and 38.3% respectively in s-Fe0, s-Fe0/BC (3:1) and s-Fe0/BC/MS (6:2:1) systems. The s-Fe0/BC/MS system was the best at nitrate removal over a wide pH range (2-12). At the same time, the nitrogen selectivity in the s-Fe0/BC/MS system was always better than that in other two systems. The X-ray photoelectron spectroscopy (XPS) results showed that the addition of manganese sand could enhance the ratio of Fe3O4 in the sponge iron surface oxide, reduce the passivation of the sponge iron surface and accelerate the electron transfer rate. As a catalyst for the corrosion of sponge iron, manganese sand can increase the corrosion of sponge iron and enhance the micro-electrolysis role between iron and carbon.