Adsorption Characteristics Of Phosphorus And Nitrogen On Drinking Water Treatment Residuals-sodium Alginate Beads

The discharge of wet sludge in China’s water supply plants is nearly 55×10⁸tons/year,and the annual production of dry sludge is about 1.5².4 million tons,but its utilization rate is far below 20%according to the statistics.In the 12th Five-Year plan Report,the disposal of water treatment residuals（WTR）were already officially put forward.In many disposal methods,the resource utilization of WTR is the best way to solve the over-distribution of WTR.However,the powdered water supply sludge are poor sedimentation performance,easy clogging,difficulty in being lost,and inability to be recycled when used in the water treatment process as an adsorbent material.Therefore,this paper choose WTR powder as the main research object,and modified it through acidification,alkalization,Salinization and thermal roasting.The pretreatment and modified WTR were embedded and fixed under the effect of SA,and the surface structure and physicochemical properties of modified WTR-SA were analyzed by XRD,FTIR,SEM-EDS,BET and other characterization methods.The factors affecting the adsorption of modified WTR-SA on ammonia nitrogen and phosphorus were discussed.The fitting analysis of the kinetic model and the thermodynamic isotherm model was combined with the characterization of the modified WTR-SA before and after adsorption.Preliminary discussion and inference of the adsorption mechanism of modified WTR-SA on ammonia nitrogen and phosphorus.The sodium chloride modified combined thermal calcination method can significantly improve the removal rate of ammonia nitrogen and phosphorus by WTR.When the concentration of sodium chloride solution is 2 mol/L,the immersion time is8 h,and the calcination temperature is 400°C,achieved the best effeciency.The ammonia nitrogen adsorption rate increased from 5.12%to 45.32%compar ed with the unmodified WTR,and the adsorption amount increased from 0.204 mg/g to 1.839 mg/g.The phosphorus adsorption rate increased from 52.01%to 72.89%,and the adsorption amount increased from 2.602mg/g to 3.645mg/g.The best embedding scheme was obtained by single factor test and orthogonal test on factors such as SA concentration,calcium chloride concentration,sludge addition amount and crosslinking time of WTR beads.The best embedding method is SA concentration of 2%,the amount of residual sludge in the water supply plant is 12g,the amount of calcium chloride added is 2%,and the crosslinking time is 6h.The prepared modified WTR-SA beads were used to adsorb 10 mg/L low-concentration nitrogen and phosphorus wastewater at pH=7 and temperature 25°C respectively.The residual ammonia nitrogen concentration of the solution after the reaction satisfies the second-class standard of surface water environmental quality and phosphorus.The residual concentration satisfies the three criteria.The characterization results show that the surface crystal structure of WTR is not changed after modification and embedding,and iron and aluminum still exist in a more active and indefinite form.After embedding WTR,a new-OH peak was produced at about 1433 cm^-1.After WTR modification,the content of C element was significantly reduced,and the proportion of adsorbed active components such as Al,Fe and Si increased.The introduction of Na⁺replaces the cations with larger radiuses such as Ca⁺and Mg⁺in WTR.The embedded WTR still has a large surface area（25.31 m²/g）and a pore volume（0.071 cm²/g）.The adsorption of ammonia nitrogen by modified WTR-SA has the best removal effect under neutral conditions,and the phosphorus has better removal effect under weakly acidic conditions.The adsorption of nitrogen and phosphorus by WTR-SA beads showed a trend of“fast adsorption and slow equilibrium”,and low concentrations of nitrogen and phosphorus could reach adsorption equilibrium faster.The adsorption of ammonia nitrogen by the modified WTR-SA beads is an exothermic reaction,and the temperature drop can promote the reaction,while the adsorption of phosphorus is opposite.Sodium hydroxide has a high regeneration rate of ammonia nitrogen,the mass loss rate after four regenerations is less than 15%.Besides,sodium hydroxide has a higher degree of regeneration rate after the modified WTR-SA beads adsorb phosphorus,and the regeneration rate is still more than 85%after four cycles of regeneration.The adsorption process of ammonia nitrogen and phosphorus by modified WTR-SA can be well described by the quasi-secondary kinetic model,and the fitting coefficient R² is above 0.995.The intraparticle diffusion fitting curves of different initial concentrations did not pass through the origin,and the adsorption of ammonia nitrogen and phosphorus by modified WTR-SA was affected by many factors.The Langmuir equation can better describe the adsorption process of nitrogen and phosphorus on modified WTR-SA.The Freundlich adsorption index n^-1 is within 0.1-0.5,indicating that modified WTR-SA is an adsorbent with excellent adsorption performance.The saturated adsorption capacity of ammonia nitrogen at 15°C,25°C,and 35°C was 2.520 mg/g,2.199 mg/g,and 2.201 mg/g by Langmuir equation.The adsorption was exothermic and the temperature decrease was favorable for the reaction;In contrast,the adsorption of phosphorus by modified WTR-SA is endothermic,and the amount of saturated adsorption increases with temperature,up to 6.447 mg/g.The adsorption of ammonia nitrogen by modified WTR-SA is mainly carried out by ion exchange and hydroxyl complexation,and ion exchange pla ys a major role therein.The adsorption of phosphorus by modified WTR-SA is affected by ligand exchange,chemical precipitation and ion exchange.The main role of ligand exchange is ligand exchange.