Effect And Mechanism Of Alginic Acid On Struvite Crystallization

The phosphorus-rich sludge is more suitable for phosphorus recovery via struvite crystallization,which not only avoids the problem of eutrophication caused by excessive phosphorus emission,but alleviates the problem of global shortage of phosphorus resources,since recovered struvite crystals are good slow release fertilizers used in agricultural production instead of phosphate fertilizer.However,the organic matter in the sludge can inevitably bring some adverse effects on the struvite crystallization,especially the large amounts of extracellular polymers（EPS）in the sludge.So far,relatively little relevant literature has been devoted to understand the effect of EPS on struvite crystallization.Due to the complexity and uncertainty of the structure and composition of extracellular polymers in the sludge,it is necessary to use analogous compounds instead of EPS.The effect of alginic acid as a representative of EPS on struvite crystallization was performed by virtue of constant composition experiment in this study,which was deployed for accurately measure struvite crystal growth rate.On this basis,the influence of single factors such as seeded struvite crystals,reactant concentration,pH and ionic strength on the inhibition of struvite crystal grow rate by alginic acid was also investigated.Moreover,the mechanism underlying the inhibitory effect of alginic acid was elucidated by means of XRD,SEM,TG-DSC,FTIR,BET,Zeta potential and potentiometric titration.To comprehensively evaluate of the adsorption behavior of alginic acid on struvite crystal surfaces,the adsorption of alginic acid on struvite crystals was investigated at various adsorption time,initial concentration of alginic acid,pH and ionic strength.Subsequently,the adsorption data were modeled as well by different kinetic and isotherm adsorption models.Importantly,the adsorption mechanism of alginic acid on struvite face was fully elucidated from the microscopic point of view by virtue of one-site and two-sites Triple Layer Model（TLM）and molecular dynamic simulation.The results may provide a theoretical foundation for obtaining high quality struvite crystals from anaerobic digestion of sludge via struvite crystallization.Alginic acid significantly reduced struvite crystal growth rate,which was attributed to the adsorption of alginic acid and subsequently blocking of active growth sites onto the surface of struvite forming rather than its complexation with Mg²⁺or the change of the ionic strength in the reaction solution.The results of FTIR and XPS analysis further revealed that the inhibitory effect was induced by the binding of Mg²⁺,P-OH on the surface of struvite crystals with COOH,C=O and C-O-C in alginic acid,respectively,whereas NH₄⁺on struvite crystal surfaces did not participate in the surface reactions.The inhibitory effect was reduced by the addition of seeded struvite crystals,increasing pH or reactant concentration and decreasing ionic strength.The adsorption of alginic acid on the surfaces of crystals was relatively fast and almost reached a plateau value within the fist 30 mins.The adsorption capacity of alginic acid obviously increased with the increase of adsorption time,the initial concentration of alginic acid and pH,but moderately decreased with increasing ionic strength.Moreover,the adsorption process followed pseudo-second order model and Langmuir adsorption isotherms model,indicative of a monolayer chemisorption.Intra-particle diffusion was the rate determining step in the adsorption process,but not the only limited stage.The adsorption of alginic acid on struvite surface was induced by the outer-sphere complexation of alginate C=O with POH and MgOH and the inner-sphere complexation of COOH with the Mg²⁺on struvite surface.One-site and two-sites CCM both showed a good fit to adsorption data as a function of pH.Subsequently,the equilibrium constants of one-site and two-sites as well as their surface site densities were determined.The results of one-site TLM revealed that the adsorption species were composed of inner-sphere（>SOHAlg^-）and outer-sphere complexes（>SAlg）,and the later dominated the adsorption,which was also confirmed by two-sites TLM,indicating two outer complexes（>POHAlg^-and>MgOHAlg^-）and one inner complex（>MgAlg）on struvite crystal surfaces.The surface adsorption energy between alginic acid and the faces of struvite crystals can overcome the strong torsional deformation,indicating alginic acid tightly binding with the crystal faces.Moreover,the adsorption energy of alginic acid with the faces of struvite crystals were found to decrease in the order:（001）>（100）>（011）>（010）>（111）>（101）.Furthermore,The results of radial function analysis of alginic acid on different crystal faces confirmed that there were strong hydrogen bond between the alginate C=O and the hydroxyl of the crystal faces.