Growth Behavior Of Struvite Crystals In Solution

Struvite crystals（Mg NH₄PO₄·6H₂O）,product of recycling solubilized P(PO₄^3-)and N（NH₄⁺）from wastewater,can be used as a slow-release fertilizer.During the industrialized production,the fine particles are too tiny to be separated from the solution,and then deposit in downstream progress blocking the pipes.Also,struvite is the main constituent of kidney stones,which can grow rapidly forming dangerous‘staghorn-calculi’in the organisms.Up to date,the morphology of struvite cannot be efficiently regulated to solve the above problems,as its growth mechanism remains unclear.In order to disclose the growth mechanism,the macroscopic and microscopic morphology of struvite has been systemically investigated under the main influence factor of pH.Detail are as follows:Macroscopic morphology of struvite was obtained under various experimental conditions.The results show that the morphology changes with the increasing supersaturation（σ）:long rod（σ<1）,star（1<σ<2.5）and dendritic crystal（1<σ<2.5）.last.Based on the diffusion-limited aggregation（DLA）model,it can be concluded that,under the higher supersaturation,the higher PO₄^3-concentration which increases with the supersaturation lowers the diffusion coefficient,impeding the diffusion process of growth units resulting in the born of dendritic crystals.While under the lower supersaturation the crystal remains long rods,and its aspect ratio deceases with pH decreasing which is attribute to the replacement of H₃PO₄ with PO₄^3-and the adsorption of H⁺on（012）face hindering the growth of the crystal’s head face.The microscopic morphology of struvite crystals was characterized by atomic force microscopy.From the surface structures of（101）and（00-1）faces grown under the fixed supersaturation,conclusion can be made.The growth mechanism of（101）face is 2D nucleation mechanism under the lower supersaturation（σ=0.9）but normal growth mechanism under the higher supersaturation（σ=0.9）,as the surface becomes gradually rougher when the supersaturation increases.While the growth mechanism of（00-1）surface remains 2D nucleation model unchanged,only transferring from mononuclear to multinuclear with the increasing supersaturation.Based on the identification of growth mechanism,growth kinetic functions of the two crystal faces are determined by correlating the experimental data.Growth mechanism of struvite was roughly revealed,in this work,through the analysis of macroscopic and microscopic morphology,which will provide a foundation for struvite growth regulation in the fields of industrial production and medical health medical treatment.