The forms of combination of Cu, Ni and Zn in anaerobic sewage sludge
As a first step in understanding the chemistry of toxic elements in sludged soil, this thesis presents a comprehensive model of their chemistry in the digester. A review of the literature had shown that heavy metals were likely to be held in 3 pools: as precipitated and detrital mineral phases 'Particulate'; as complexes with the flocculated biomass 'Biofloc'; and as complexes in solution 'Soluble'. A simple pragmatic fractionation procedure has been offered to separate these 3 pools based on their physical properties in water. A mass-balance between the pools showed that the 'Particulate' fraction held only 5-16% of the heavy metals but contained them in the highest concentration. The 'Biofloc' held 82-94% of the heavy metals. The 'Particulate' material was subdivided by density separation and examined by a combination of analytical SEM and XRD. Thirty-four minerals were identified by XRD, many of which were detrital. Secondary precipitates on the surface of detrital minerals were revealed by the SEM; of these only the sulphides were found to contain detectable levels of heavy metals. Eleven minerals were identified in the 'Biofloc', of which most were clays. The fractionation scheme defined 2 fractions that could hold heavy metals by complexation. The 'Soluble' had a CEC of 8.8 meq/gm and the 'Biofloc' 4 meq/gm. Complexation by the heavy metals and a few other important cations was measured. A thermodynamic model was built which describes the possible solution species, mineral phases and complexation by the biomass in terms of a set of 33 primary components. This model was solved by computer for an 'average' sludge based on published analyses, and considered 313 solution species, 42 exchange reactions and 129 possible minerals. The predicted speciation was broadly in line with observations and suggested that the majority of the heavy metals separating with the 'Biofloc' would have been held as fine enmeshed sulphide precipitates. The model may be used with existing programs such as MINEQL and GEOCHEM. Preliminary studies have shown that with a few additions the model may be used to describe the heavy metals in sludged soil.