Study On The Processes And Mechanisms Of Vivianite Recovery

The modern urban drainage system has altered the original cycle of phosphorus in the era of ecological civilization,resulting in phosphorus crisis.Most of the phosphorus that enters the human body via food will eventually accumulate in the waste sludge?⁹0%?during sewage treatment,becoming the"second phosphate mine"to be explored.During the anaerobic digestion process,the phosphorus and iron will form vivianite?Fe₃?PO₄?₂·8H₂O?with the presence of specific microorganisms.Vivianite has a larger value than being used as fertilizers.Thus,the present study explores the mechanisms of vivianite formation from the aspects of Fe³⁺bioreduction and environmental factors in the anaerobic digestion of waste sludge.Based on literature review,the study first focused on how to increase the rate of Fe³⁺bioreduction in order to enhance the vivianite formation.Therefore,NH₄⁺and SO₄^2-were added to the reactor to stimulate the bioreduction of Fe³⁺.The experimental results showed that the addition of NH₄⁺and SO₄^2-did not increase the bioreduction of Fe³⁺,indicating that the Metal-reducing bacterias in the system already have a higher activity and is not a limiting factor for Fe³⁺reduction.However,the oxidation of NH₄⁺is coupled with Fe³⁺reduction.This autotrophic Fe³⁺reduction process caused a significant increase in dissolved organic matter?²3%?in the reactor,resulting in a 17%increase in methane production.Meanwhile,the NH₄⁺oxidation consumed H⁺in anaerobic digestion system,resulting in an increase in pH,and weakened the binding ability between Al³⁺and PO₄^3-.This invisibly increased the chance of Fe²⁺and PO₄^3-combination to form vivianite,and increases the production by 19%.With the help of the diversity and high activity of microorganisms in the anaerobic digestion system,the influence of the chemical structure and physical properties of iron sources?hematite,goethite and amorphous iron ore?on Fe³⁺reduction and vivianite formation was explored.At the same time,the feasibility of using these iron sources for vivianite was evaluated.The results showed that the reduction degree of Fe³⁺in amorphous iron ore,goethite,and hematite sequentially decreased.This is because the crystallinity of iron limited the access of microorganisms,which in turn affected the formation of vivianite.Phosphorus fractional experiment results showed that unstable phosphorus is the potential phosphorus source that can combine with iron to form vivianite in anaerobic digestion system.Theoretically,the solubility and supersaturation of vivianite crystal in the water phase are affected by temperature.However,the low-temperature,medium-temperature and high-temperature anaerobic digestion experiments showed that the precipitation of vivianite did not make a significant difference.The simulation calculation supported the experimental result that the precipitation of vivianite in the temperature range of 5⁹5?was less affected by temperature.