Study Of Adsorption Efficiency Of Water Treatment Residual Sludge For Ammonia Nitrogen And Fluoride

For resource utilization, water treatment residual sludge (WTRS) and its modifiedproducts were used as adsorbents to remove NH4+and F-from aqueous solution. Theadsorbent materials were characterized by EDX, XRD, FTIR, SEM, TG-DTA and BETspecific area test techniques to understand their nature characteristic and externalconstruction. Different experimental conditions such as simulated wastewater pH, thedosage, initial NH4+concentration, the temperature and time of adsorption wereinvestigated in detail. The experimental data were fitted to adsorption isothermal modelsand kinetic models, respectively, to illustrate the nature of NH4+and F-onto adsorbentsmaterials.Results of the characteriaction show that WTRS was a silicate mineral which was richin metal elements such as Si, Al, Fe, Ca, Mg, K, C, O and so on. The moisture and volatilesof WTRS were removed by roasting at high temperature. The alkali modified watertreatment sludge had a rough surface with large specific area. Whereas, the structure andcomponent of the water treatment sludge have no obvious change after cerium modified.Results of the NH4+adsorption analtsis demonstrate that the appropriate adsorptioncapacity is achieved when the125~150μm of WTRS particles, which have been roasted at500℃for3h, were used. Alkali modified sludge has shown the greatest potential forammonia nitrogen removal. The uptake of ammonia nitrogen by alkali modified WTRS is2.13times larger than unmodified adsorbent when the concentration of NaOH was chosenas5%. It was indicated that in weak acid condition the final concentration of NH4+insimulated wastewater could reach the secondary standard of ammonia emissions standardafter120min adsorption at room temperature at the dosage of20g/L for the initial NH4+concentration of50mg/L. It was found that pseudo-second order kinetic model were thebest-fit. Further analyses of intra-particle diffusion model indicate that surface diffusionand intra-particle diffusion were both the rate determination step. The data of adsorptionisotherm tend to reveal that the adsorption process should fit the Langmuir and Temkinisotherm modles. The maximum adsorption capacity (qm) obtained from Langmuirisotherm was2.32mg/g, which38%higher than unmodified sludge. The adsorptionmechanism of NH4+was proved to be electrostatic attraction and ion-exchange.Results of the F-adsorption analtsis demonstrate that the appropriate adsorptioncapacity is achieved when the125~150μm of WTRS particles, which have been roasted at 500℃for4h, were used. Cerium modified sludge has shown the greatest potential forfluoride removal. When the concentration of Ce(NO3)3was chosen as2%, the uptake offluoride by cerium modified WTRS was abve95%after120min adsorption at roomtemperature at the dosage of10g/L for the initial F-concentration of50mg/L in the pHrange of3~9. Good fitting of pseudo-second-order kinetics model indicated thechemisorption nature. Further analyses of intra-particle diffusion model indicate thatsurface diffusion and intra-particle diffusion were both the rate controlling step. It wasfound that the Langmuir and Temkin isotherm model fitted the experimental data better.The maximum adsorption capacity (qm) determined by Langmuir isotherm model was14.52mg/g for Ce-WTRS,24.5%higher than WTRS. Ion exchange and complex reactionwere identified as the main mechanism of fluoride adsorption.The great potential of WTRS for NH4+and F-removal reveal that WTRS can beapplied efficiently as a low-cost sorbent for resource utilization.