Ferric precipitation of phosphate is used extensively in wastewater treatment processes worldwide, including Australia. Additionally, ferric is also used as coagulant in water treatment processes. In all cases, large amounts of ferric sludge (comprising ferric oxy-hydroxides and FePO4) are produced, that are typically stockpiled or disposed of in landfills. Very limited beneficial reuse options are available for this material at present. The global need of new sources of nutrients and clean water has motivated the search of novel and economically viable processes for the treatment of wastewaters and the recovery of nutrients found therein. The use of sulfide to form iron sulfide precipitates (a mixture of mackinawite (FeS) and pyrite (FeS2), denoted as FeSx) is an attractive alternative to existing options for separation and recovery of phosphorus from ferric phosphate sludge generated in wastewater treatment. This thesis demonstrates that electrochemical oxidation of FeSx can then be utilized to recover ferric iron for reuse back in the phosphate removal process.
This thesis aims to prove the concept of a novel 2-stage process for iron and phosphate recovery from ferric phosphate sludge. In Stage I, the phosphate is released in solution after the addition of a sulfide solution via FeSx precipitation. In Stage II, an electrochemical reaction with oxidation of FeSx to sulfur on an anode enables the recovery of iron with the simultaneous cathodic release of sulfide, which can be reused in the process. In particular, the goals of this work are: (i) to identify and study the factors affecting the FeSx formation from synthetic and real ferric sludge and its precipitation by gravity settling, in order to maximize phosphorous recovery with an economical process; (ii) to determine whether the FeSx particles formed in the sulfide addition experiments are reactive for anodic oxidation on carbon based electrodes; (iii) to demonstrate the feasibility of the proposed novel electrochemical process for the recovery of soluble iron and sulfide from FeSx sludge.
In order to elucidate the reaction chemistry and to understand the mechanisms of precipitation of the inorganic iron sulfide particles, most experiments in this work were carried out using synthetic FePO4 sludge, as a way to eliminate potential interference of organic solids, which are invariably present in ferric phosphate sludges obtained from wastewater treatment. However, experiments were also run with industrial ferric sludge from a drinking water treatment plant to demonstrate the process on the more complex material. Cyclic voltammetry integrated with liquid phase sampling was carried out to determine the sequence of electrochemical reactions, and X-ray absorption spectroscopy (XAS) was used to determine the speciation of both Fe and S in the FeSx sludge as a function of time, to understand the FeSx transformations related to changes in the reactivity with time. Electrochemical monitoring and liquid phase sampling enabled the calculation of iron and sulfur balances in a lab-scale model of the integrated 2-stage process.
This work identified that the S:Fe molar ratio and pH are the key factors that affect the phosphorus recovery, as well as the optimal process conditions to achieve an effective solid-liquid separation. Effective phosphorus recovery from real ferric phosphate sludge was achieved by sulfide addition, reaching 66±5% phosphorus recovery at a S:Fe stoichiometric molar ratio of 1.5 and increasing up to 95±4% P as the S:Fe molar ratio increased to 2.5. The point of charge zero of the FeS particles was found to be at pH 4, leading to the efficient coagulation and settling of the particles at this pH.
This study proved for the first time that FeSx particles are reactive for anodic oxidation on graphite electrodes. The reactivity of the FeSx particles gradually decreased with time after FeSx precipitation. The change in the anodic reactivity of the FeSx sludge with time is a result of changes in the chemical speciation of the FeSx sludge, i.e. the greater anodic reactivity of the fresh FeSx sludge is a result of mackinawite (FeS) being the predominant compound. Conversely, the pyrite (FeS2) that is formed over time is likely responsible for the reduced anodic reactivity of the FeSx sludge.
The feasibility of electrochemical recovery of soluble iron was also proven with a lab-scale two-compartment electrochemical cell run in batch mode. Up to 60% Fe and 55% S (as sulfide/polysulfide) were recovered in the anodic solution and cathodic solution, respectively, at the applied anode potential of +0.8 V vs. SHE on graphite granules. Peak current densities of 9.5 ± 4.2 A m-2 and power requirements as low as 2.4 ± 0.5 kWh kg Fe-1 were reached with real full strength FeS suspensions. With the findings of this work, the cost-effective recovery of iron and phosphorus from ferric phosphate sludge via the 2-stage integrated process becomes a step closer to reality. However, further research is required to test different additional electrodes, in order to increase current densities and Fe and S recoveries, to reduce the operating costs and hence achieve the economical feasibility of the integrated process.