Recycling of phosphorus-recovered incinerated sewage sludge ash for stabilization/solidification of contaminated marine sediments

Abstract

To progress our cities towards sustainable waste management, an array of reuse and recycling methods have been developed to manage the growing amounts of incinerated sewage sludge ash (ISSA) from the combustion of dewatered sewage sludge. Phosphorus (P) recovery from ISSA via the cost-effective and highly efficient acid washing method (with H2SO4 as the leachant) has been increasingly acknowledged as a critical step to resource efficient recycling of ISSA. However, it generates a residual ash i.e., P-recovered ISSA, accounting for over 80% by mass of the original ISSA. The aim of this study is to explore reuse options for the residual ash so that a complete 'close-loop' recycling of ISSA is achievable without the generation of waste. Given the rich iron content and insignificant amounts of toxic heavy metals in the P-recovered ISSA, the focus of this study has been directed to reuse it as a low-cost supplementary iron source to synthesize iron-containing functional materials for the remediation of heavy metal(loid) contaminated waters and dredged marine sediments. In this thesis, co-pyrolysis of the P-recovered ISSA with a model biomass material i.e., lignin, for aqueous Cr(VI) removal was firstly explored. The optimized co-pyrolysis process successfully turned most of the iron oxides in the P-recovered ISSA residue into zero-valent iron (ZVI) and improved surface properties of the end composite material that rapidly and effectively removed Cr(VI) from an acidic solution primarily via chemical reductions. In order to extend the scope of feedstock biomass and expand the application horizon, another typical biowaste i.e., peanut shell was then co-pyrolyzed with the P-recovered ISSA for the removal of As(III) and As(V) from aqueous solutions under both acidic and near neutral environments. The effect of silica on the pollutant removal by the resulting composite material was addressed as well considering its dominant presence in the P-recovered ISSA. Silica in the P-recovered ISSA impeded the formation of ZVI and magnetite by forming iron silicon during the co-pyrolysis process and resulted in notably increased leaching of reactive silica during the sorption process that competed with arsenic for active adsorption sites. The produced sorptive materials derived all from solid waste still exhibited satisfactory removal performance for heavy metals from aqueous under optimized conditions making the co-pyrolysis method attractive for the management of the P-recovered ISSA. To facilitate finding an end use of the spent material after the removal of pollutants, shaping of the powdery mixture of the P-recovered ISSA, peanut shell and waste bentonite with a mass ratio of 1:1:2 to pelletized granules was achieved via sintering under a nitrogen atmosphere. The end product was produced mechanically strong (4.9 MPa) and lightweight (1.21 g/cm3) millimeter-sized granules. These granules could remove As(V), Cr(VI), Cu(II) and Pb(II) from aqueous solutions via mechanisms including redox reactions, complexation and (co)precipitation with the corresponding maximum adsorption capacity at 14.11, 5.79, 14.12 and 23.52 mg/g. The spent granules could be environment safely upcycled as a partial river sand replacement in cement mortars based on the Toxicity Characteristics Leaching Procedure. The powdery composite material produced from the co-pyrolysis process could also be used as a green additive in cement-based stabilization and solidification processes that achieved an enhanced immobilization and recycling of As-, Cr- and Cu-contaminated marine sediments into useful construction materials. Thus, the aim of this thesis was realized by providing a technique for 'close-loop' recycling of ISSA.