Soil solution sulfide control by two iron-oxide minerals in a submerged microcosm

Under reduced conditions, commonly experienced at the bottom of aquaculture ponds, sulfate is reduced to sulfide. Sulfide in the form 'H2S' is highly toxic to aquatic organisms. Iron oxides have the capacity to regulate the formation of sulfide by poising the redox sequence and to form insoluble iron sulfide compounds, further decreasing the activity of sulfides in solution. Ferrihydrite, a poorly crystallized, high surface area hydrous iron oxide, and hematite, a highly crystallized, low surface iron oxide, were separately added to a synthetic soil composed of bentonite, kaolinite, and quartz sand in a microcosm study. Bermuda grass was added to the soil as an organic matter source to induce reducing conditions. The soils were flooded with artificial seawater diluted to salinity levels of 1, 14, and 34 ppt. Concentration solutions of total dissolved sulfide and iron in interstitial; and of oxygen and pH in the water column were measured 25, 46 and 72 days after flooding the soil. Concentrations of sulfide in treatments with hematite and ferrihydrite were significantly lower than those of treatments without iron oxides. On all sampling dates, ferrihydrite was up to eight times more effective than hematite in regulating the accumulation of sulfides in solutions with salinity levels of 14 and 34 ppt. Mean dissolved-oxygen concentrations in treatments with ferrihydrite were higher than those of treatments with hematite. At the intermediate salinity level, adding either hematite or ferrihydrite at rates higher than 4 g kg(-1) did not significantly improve the effectiveness of sulfide removal from solution. At the highest salinity level, increasing the concentration of hematite from 4 to 13 g kg(-1) may have some practical merit, while higher rates may not be justifiable. There appears to be no advantage in applying ferrihydrite at rates higher than 4 g kg(-1) at the highest salinity level. (C) 2004 Published by Elsevier B.V.