Highly Efficient Removal Of Phosphate From Aqueous Solution By Bio-Nanocomposite-based Materials And Its Mechanism

Nutrient pollution is closely related to the increasing nutrient content(especially phosphate)in water bodies.Nutrients are the underlying cause of surface water eutrophication,which facilitates algal development and oxygen depletion through enriching nutrients(usually nitrogen or phosphorus).However,researchers have needed special attention for phosphate removal from water.There are so many phosphate contamination resources such as farm and field surface drainage,septic tank discharges and dairy farming,fire waste and depletion of biodiversity.Various materials have been used for wastewater treatment,such as graphene,agriculture waste,nanomaterials,carbon nanotubes,clay minerals,montmorillonite,etc.Agriculture waste collected from any rural area which plays an important role in various environmental remediation,for example,wastewater treatment process,enhancing the soil fertility and mitigating climate changes etc.In this Ph.D.dissertation,a variety of agricultural waste-based nanocomposites materials were successfully prepared by a one-step method at 200?,and the removal of phosphate in water by adsorption was studied.The physical and chemical properties of the prepared target adsorbents were investigated by modern instrument analysis and methods,and the types of surface functional groups of the bioadsorbents were determined by Boehm titration.Phosphate removal capacities were determined by carrying out various adsorption tests at numerous conditions such as,various pH ranges,different concentrations at different temperatures,multiple time intervals with presence of different co-existing anions and organic matter.The main research results of this paper are summarized as follows:(1)First,Zr-La bimetallic supported biomass adsorbent was successfully prepared in the experiment and used for phosphate adsorption removal.In the research process,separate Zr(OH)4 was loaded on the pomegranate peel to prepare Zr/Peel adsorbent,and Zr(OH)4 and La(OH)3 composite bimetals were loaded on the pomegranate peel to prepare Zr-La/Peel adsorbent.The material was characterized by scanning electronic microscopy(SEM),X-ray photoelectronic spectroscopy(XPS)and X-ray diffraction(XRD).The results showed that Zr and La were successfully loaded onto the surface of pomegranate peel.In batch adsorption experiments,the adsorptions efficiency of Zr-La/Peel was better than Zr/Peel adsorbent at the identical pH value and the adsorption efficiency of phosphate declined with the increase of pH value.At the same time,the pH experiment results show that the hydroxides of La and Zr were released a large number of hydrogen ions during the hydrolysis reaction,resulting in protonation and electrostatic attraction.The Zr-La/Peel adsorption data for phosphate removal can be well fitted by all isothermal models,for example,Langmuir,Freundlich,Redlich Peterson,and Sips isothermal models.Zr-La/Peel showed the best phosphate adsorption capacity at 40?(53.414 mg/g),and the pseudo-secondary-order kinetic model well fitted the process of phosphate adsorption.The coexistence of Cl-does not affect the phosphate removal efficiency by adsorbent,while the presence of SO42-,NO3-humic acid(HA)reduces the adsorption capacity of phosphate by 20-30%.In order to determine the Zr-La/Peel recycle,four intermittent adsorption-desorption cycles were carried out.Due to the leaching of metal ions of Zr-La/Peel in each cycle,the regenerated properties of the adsorbent decreased continuously.Subsequently,after 1st regeneration cycle,lixiviate rates of Zr and La were 43 ug/L and 4 ug/L,respectively.After two cycles of regeneration,the adsorption efficiency of Zr-La/Peel remained above 82%,and after four cycles,the adsorption capacity was 58%.Therefore,the adsorbent still needs to be further modified to improve its phosphate removal efficiency.(2)Secondly,a biological adsorbent(Fe-La/Peel)for dephosphorization was prepared by loading dual metallic Fe-La nanoparticles onto pomegranate peel fibers using a solvothermal synthesis process.The fresh and spent adsorbents were characterized by SEM,XPS,FTIR,and other modern analytical instruments and methods.The results showed that the specific surface area of Fe-La/Peel was 93.18 m2/g,and the surface was rough and evenly enclosed with Fe and La,which provided a adsorption capacity site for phosphorus.Boehm titration showed that the carboxyl characteristic site of Fe-La/Peel was 1.521 mmol/g,which was higher than that of La/Peel(0.9251 mmol/g).At the same pH value,the removal efficiency of Fe-La/Peel was higher than that of La/Peel and Fe/Peel,and the adsorption capacity of phosphate was higher under neutral conditions,and the dissolution amount of metal was almost negligible.In the batch adsorption experiments,the maximum adsorption capabilities of Fe-La/Peel were calculated by Langmuir and Sips isothermal adsorption models as 75.09 mg/g and 78.99 mg/g,correspondingly.The average minimum metal dissolution of Fe-La/Peel was 0.06 ?g/mL under alkaline conditions,while the maximum metal dissolution of Fe-La/Peel was 8.13 ?g/mL at acidic conditions.The phosphate adsorption by Fe-La/Peel bio-nanocomposites was uniform,and the adsorption capacity gradually increases with the rise of temperature.The phosphorus removal capability of Fe-La/Peel was also related to the concentration of coexisting anions(C1-,NO3-,HCO3-).In the presence of an electrolyte,the adsorption efficiency was reduced lower than 20%.After regeneration,the phosphate removal rate of Fe-La/Peel and La/Peel decreased to 58.9%and 56%,respectively,but after five cycles of adsorption and desorption,the adsorption efficiency of Fe-La/Peel and La/Peel for phosphate was still up to 28%and 16.8%.The reason for the low regeneration efficiency of bioadsorbents was that the dissolution of metals(Fe and La)decreased the number of active sites on the biological adsorbents.Adsorption experiments showed that Fe-La/Peel biological adsorbent had a good adsorption capacity for phosphate.The above research results indicate that Fe-La/Peel has a great application prospect in the extraction of phosphate from wastewater.(3)Finally,the bimetallic Ni/La nanoparticles were doped onto pomegranate fibers(Ni-La@Peel)by solvothermal synthesis method to prepare a new biological adsorbent,which was used in the study of phosphorus removal.SEM,XPS,FTIR and other instrumental analysis methods were used to characterize the adsorbents before and after adsorption.The results showed that the specific surface area of Ni-La@Peel was 31.84 m'/g,and the surface was rough and evenly coated with Ni and La,which provided adsorption sites for phosphorus removal.Under four pH values(3.78,4.44,5.60 and 6.68),the maximum phosphorus removal rate of the adsorbent was over 96%.In the batch adsorption test,the Langmuir model calculated the maximum adsorption capacity of phosphate at Ni-La@Peel as 226.55 mg/g(25?).At the same time,the carboxyl content of Ni-La@Peel was determined by Boehm titration as 3.15 mmol/g,which was higher than that of other similar adsorbents.All the results were consistent with the Langmuir isothermal model(R2:0.99)and kinetic pseudo-secondary order models(R2:0.99),indicating that the removal mechanism of phosphate by Ni-La@Peel is mainly related to homogeneous chemisorption.The experimental results of co-existing anions showed that in the presence of other anions(such as chloride,sulfate,nitrate,bromide and fluoride),the adsorption capacity of phosphate was only reduced by about 10%compared to the previous condition.In addition,the phosphorus removal efficiency of Ni-La@Peel remained above 80%after 7th regeneration cycles.The results showed that Ni-La@Peel is a promising bioadsorbent for wastewater treatment.The results show that Ni-La@Peel nanocomposite had a good phosphorus removal efficiency.