Study On The Mineralization Mechanism Of Biogenic Struvite

Biomineralization is a common phenomenon in nature. Compared with abiogenic minerals, biominerals produced by biomineralization usually possess unique composition and morphologies, complex assembled architectures, and excellent mechanical property due to the rigorous regulation by specific biomacromolecules/organic macromolecules and biological activity in vivo during the mineralization process. However, when the mineralization process in vivo loses control, pathological biomineralization occurs, leading to the formation of various stones and a series of diseases. Hence, the medical researchers need to understand pathological biomineralization mechanisms to deal with the related diseases and develop effective treatment methods. This gave birth to the medicine mineralogy. As a kind of phosphate mineral, struvite is not only a natural biomineral, but also the main component of urinary calculus. Meanwhile, struvite is a slow-release fertilizer in agriculture and a kind of scale often seen in sewage treatment plants. Struvite plays different roles in different environments, and has obtained considerable attentions of various fields including biology, medical science, chemistry, environmental science and mineralogy. In this dissertation, we combined the biomineralization and biomimetic mineralization methods to systematacially investigate the microcosmic mineralization mechanisms of different biogenic struvite in molecular level. We first explored the molecular mechanisms of microbially induced struvite formation and microbially mediated struvite morphogenesis, and revealed the effects of different bacterial components on struvite morphology. At the same time, model peptide was employed to investigate the effect of urinary proteins intimately involved in the urinary stones formation on the crystallization and morphological evolution of struvite, and the inhibition mechanism of urinary proteins on struvite growth was illuminated. On this basis, the discovered mechanism that proteins can inhibit struvite growth was applied to search for effective chemical inhibitors and develop new antiscale techniques. The obtained results can not only deepen the insight into biomineralization and pathological mineralization of struvite, but also provide a new way to lithiasis treatment and scale control. Several important results of this dissertation were summarized as follows:1. Bacterially mediated struvite usually crystallizes as unusual morphologies. To better understand the relationship between growth habit of struvite and bacterial activity in struvite biomineralization process, Shewanella oneidensis MR-1 was selected as a model microbe to induce struvite mineralization in the artificial wastewater. Meanwhile, different bacterial components were isolated from the cultures by a set of separation techniques, to study their influence on struvite crystallization, growth and morphology formation. Bacterial mineralization experiments demonstrated that S. oneidensis MR-1 could not only induce mineralization and growth of struvite, but also mediate the formation of coffin-like struvite. Biomimetic mineralization experiments revealed that different bacterial components had different effects on struvite morphology. That is, LMW peptides played a key role in the formation of coffin-like struvite while the formation of prismatic or truncated tabular struvite might be controlled by the SEPS or the BEPS. Bacterial mineralization experiments also revealed that strain MR-1 can not only induce struvite mineralization, but also directly transform organophosphorus and organic nitrogen into struvite. Current results can provide a deeper insight into bacterially mediated struvite morphogenesis, and a new way to recover phosphorus and nitrogen from wastewaters and eliminate eutrophication.2. Struvite stone is a serious clinical medical problem, which has plagued human beings for a long time. Recent studies have found that certain urinary proteins can efficiently inhibit struvite stone formation. These discoveries are significant for developing effective therapies for stone disease, but the inhibition mechanism of crystallization remains elusive. Some studies reported that struvite with special morphology formed in the presence of urinary proteins, and the mechanism remained to be clarified. Hence, polyaspartic acid (PASP) was employed as a model peptide to investigate the effect of urinary proteins on the crystallization and morphogenesis of struvite in ammonia diffusion system. The results demonstrate that selective adsorption/binding of PASP onto the {010} and {101} faces of struvite crystals results in arrowhead-shaped morphology, which further evolves into X-shaped and unusual tabular structures with time by oriented attachment, fusion and growth. Noticeably, these morphologies are reminiscent of biogenic struvite morphology. However, in the absence of PASP, only rod-shaped or long irregular tabular crystals were harvested. In the presence of aspartic acid, only plenty of long irregular tabular struvite crystals can be obtained. This showed that the aspartic acid monomer cannot exert a similar influence as PASP. Concentration-dependent experiments show that PASP can inhibit struvite growth and the inhibitory capacity increases with increasing PASP concentration. Considering that PASP is a structural and functional analogue of the subdomains of aspartic acid-rich proteins, our results reveal that aspartic acid-rich proteins play a key role in regulating biogenic struvite morphology, and aspartic acid residues contribute to the inhibitory capacity of urinary proteins. Based on the good biocompatibility and efficient inhibitor capacity, PASP can potentially be developed into a therapeutic drug for urinary stone disease. As such, this study provides a new insight into the pathological biomineralization and a path toward the eventual design of therapeutic agents for urinary stone disease.3. The struvite scaling appeared in the wastewater treatment processes causes a range of operational problems and obstructs the normal operation of sewage treatment plants badly. Therefore, many efforts have been made to develop effective chemical inhibitors for struvite scale. Based on our previous study, we know that urinary proteins can effectively inhibit struvite stone growth by aspartic acid residues. This mechanism can be used to prevent struvite scaling in wastewater system. Here, the inhibitory capacity of PASP on the spontaneous precipitation of struvite at pH 9 in synthetic sludge liquor was investigated. The precipitation experiments dosed with PASP unveiled that PASP is effective in growth inhibition of struvite and its inhibitory capacity is proportional to its concentration, and that PASP also plays a role in the morphological modification of struvite crystals. The effect of several key physicochemical parameters, including pH, mixing energy, reaction time, and calcium ions on PASP inhibition performance was examined for potentially practical application. The results showed that PASP could still exert strong antiscaling on struvite even in a long running time and a large range of mixing energies. However, its inhibition potency is pH dependant, and decreases with pHi. The presence of calcium ions slightly reduced the inhibition potency of PASP while the dose of PASP prevented the formation of amorphous calcium phosphate. Dissolution experiments dosed with PASP showed that PASP could promote the dissolution of preformed struvite and its effectiveness increases with concentration. It appears that PASP has a strong ability to not only effectively inhibit the formation and growth of struvite, but also facilitate struvite dissolution. Therefore, PASP can serve as an environmentally friendly scale inhibitor and scale cleaning agent.