Water Journal : Water Journal November 2012-1
odour management refereed paper technical features 50 NOVEMBER 2012 water Step 1. When the unit is initially commissioned the surface of the iron packing material is converted from iron to rust. This reaction initiates the process. Step 2. The rust is converted to iron (II) sulphide when it comes in contact with the hydrogen sulphide in the foul air. Step 3. The rust layer is then regenerated by the reaction with oxygen in the air, leaving elemental sulphur attached to the media. The chemistry of the process is detailed below. STEP 1: Rust Formation on Iron In the presence of oxygen and water a series of internal galvanic cells or batteries is created on the surface of carbon steel. The carbon impurities become the site of reduction. Reduction half equation: 4e- + 2H2O(l) + O2(g) → 4OH- (aq) Oxidation half equation: Fe(s) → Fe2+ (aq) + 2e- When the Fe2+ (aq) and OH- (aq) ions meet they combine to produce the precipitate, iron (II) hydroxide Fe(OH)2: Fe2+ (aq) + 2OH- (aq) → Fe(OH)2(s) Fe(OH)2 is further oxidised in the presence of air and finally hydrated to produce rust. Fe(OH)2(s) +1/2O2(aq) + H2O(l) → Fe2O3.xH2O(s) (rust) STEP 2: Reaction of Rust with Hydrogen Sulphide When rust is placed in direct contact with hydrogen sulphide gas, the iron oxide is reduced to form iron (II) sulphide in the following reaction: Fe2O3.xH2O(s) + 3H2S(g) → 2FeS(s) + 3H2O(l) + S(s) It has been stated by some texts that iron (III) sulphide (Fe2S3(s)) is also formed. Iron (III) sulphide is a solid, black unstable powder which does not occur in nature. Should it be formed it will decay rapidly at ambient temperature into a yellow-green powder as per the following reaction: Figure 1. View of RICF roof showing wash system. Figure 2. Media being pre-rusted prior to commissioning. Figure 3. P&ID Sydney Street RICF installation. Figure 4. General view of RICF and booster fan.
Water Journal December 2012
Water Journal September 2012-1