La-Modified Magnetic Biochar As Novel Adsorbents For Efficient Phosphorus Removal And Nutrient Recycling

Phosphorus is a key nutrient for plant growth,and also a crucial nutritional ingredient for the development of living organisms in the aquatic system.However,problems arise when the excess phosphorus from agricultural run-off and the municipal wastewate r from discharge runs into water bodies.This may cause eutrophication,leading to the growth of algae bloom,water quality deterioration,death of fishes and ultimately the collapse of the aquatic ecosystem.Therefore,phosphorus elimination is vital for the remediation of eutrophication to avoid deterioration of water quality.Various studies have recognized phosphorus as an important factor of eutrophication in most lake ecosystems.Phosphorus used as fertilizers and domestic use such as detergents can contribute to human development.This can inevitably lead to the release of phosphorus into the reservoirs,lakes,rivers and streams.Reducing the amount of phosphorus in the aquatic ecosystem is the key theme to solving the problem of eutrophication of water bodies.Over the years,so many methods,adsorbents have been employed for the means of reducing and controlling the aftermath of phosphorus pollution in waterbodies.Among them,adsorption has great potential,such as easy operation,low cost,select ivity and effectiveness of low-concentration pollutants.The use of lanthanum-based adsorbents has proven to be very effective for phosphorus removal.It has a strong affinity for phosphorus and has great potential..However,Nanoparticles of lanthanum ten d to aggregate in water when used as an adsorbent,thereby leading to low utilization efficiency of pure lanthanum during the cleanup of phosphorus.Thus,there is a need to incorporate lanthanum nanoparticles into a suitable carrier to overcome this challenge of aggregation.We consider the use of biochar as an appropriate carrier for lanthanum.Biochar has been reported as a good carrier because it matrix allows the incorporation of other elements.In the application of biochar for water treatment,the main limitation is its tiny particle size and small density,which makes it complicated to separate after usage.To eliminate or reduce this issue,the fabrication of magnetic nano-adsorbents has recently been researched.This helps to recovery the adsorbe nts,thereby removing the problem of particle leaching and encourages the reuse of adsorbent materials.Metal fabrication of adsorbents can also aid the crystalline structure,surface herterogenity,porosity,thermal stability and yield of such adsorbents.Owing to such significant benefits as economic property,smooth operation and high efficiency,magnetic separation has been used together with many adsorbents for decontamination of phosphorus-containing water with efficient recovery.However,few researc hes have been carried out for the combined utilization of magnetic separation and porous biochar supported La to enable dephosphorization..In incorporating of the iron and lanthanu m into the matrix of the biochar the hydrothermal method was considered.it is among the most practical and labour-saving method for fabricating La-nanomaterials because it helps to neglect the use of special instruments and complicated techniquesPreparation of adsorbents:Herein,relatively cheap material and chemical agents were employed for adsorbent fabrication;two sets of adsorbents were fabricated through pyrolysis of corns straw,the porosity of the adsorbents was ensured through the use of potassium bicarbonate（KHCO₃）activation;Lanthanum and property of magnetization（Iron-Fe）were introduced into the adsorbents through one and two-steps hydrothermal method.The two-step hydrothermal method produced an adsorbent named La（OH）₃ decorated magnetic porous biochar（La₃-MPBC）,while the one-step hydrothermal method produced an adsorbent named Na La（CO₃）₂ decorated magnetic biochar（La-Fe-BC）.The adsorbents were characterized and used for batch experiments.Finally,based on the better performance of La-Fe-BC for phosphorus uptake,it was selected for a bioassay experiment in order to evaluate its capacity for bio-fertilizer on soil using maize plant as a test crop.Characterization of adsorbents:The S_BET of the biochar was 1160.736 m².g^-1,a clear indication of KHCO₃ activation.After loading of the nanoparticles of iron and lanthanum,La₃-MPBC recorded a higher the S_BET of 287.40 m².g^-1 while La-Fe-BC had 20.61 m².g^-1.The magnetism of the two adsorbents were close,9.1 and 9.01 emu.g^-1 for La₃-MPBC and La-Fe-BC,respectively.It was found that high magnetization separation efficiency of more t han 93%for La₃-MPBC and 91%for La-Fe-BC was achieved within 3 min.The X-ray powder diffractometer（XRD）revealed a covered structure of Fe ₂O₃ and La（OH）₃ on La₃-MPBC while that of the covered structure on La-Fe-BC was Fe₃O₄ and NaLa（CO₃）₂.N₂ adsorption-desorption and pore distribution experimental results showed that prior to the addition of Fe and La,the ordinary porous showed mesopores and macropores,and illustrating the TypeⅠ BET adsorption isotherm.However,after the addition of Fe and La,the adsorbents followed the same trend,consisting of both mesopores and macropores while demonstrating Type Ⅱ BET adsorption isotherm.FTIR showed the peaks relating to the C=C on La₃-MPBC and peaks relating to Fe,OH,La on both La₃-MPBC and La-Fe-BC,while after adsorption of phosphorus,the peak related were found on the adsorbents indicating the successful adsorption.Noticeably,the structures of the adsorbents did not change after loading the nanoparticles,which indicated the stability of the adsorbents.This was confirmed by the X-ray photoelectron spectroscopy（XPS）analysis for the two adsorbents.The batch study revealed a fast kinetic with adsorption equilibrium of 150 min for La₃-MPBC,Avrami fractional-order giving the best-approximated phosphate uptake with an adsorption capacity of 116.08 mg.g^-1,best fitted by Langmuir model for La₃-MPBC.La-Fe-BC had a slower adsorption kinetic of 6 h,best fitted wit h pseudo-second order.However,La-Fe-BC had a higher adsorption capacity of 330.86 mg.g^-1.Both adsorbents exhibited the endothermic characteristic of phosphorus uptake as expressed by the positiveΔH°andΔS°and negativeΔG°indicating a spontaneous reaction.The phosphorus adsorption capacity of La₃-MPBC was across a pH range of 3.0-6.0 while it was substantially stable during the interference of various co-existing ions with over 92%of phosphorus removal and 77%desorption efficiency maintained after four recycles.The kinetic La-Fe-BC was much slower than La₃-MPBC（24 h）.However,it exhibited an excellent monolayer p hosphorus uptake of330.86 mg.g^-1 over a wide pH range of 3.0–8.0,undisturbed affinity to phosphorus in the presence of various co-existing ions(e.g.NO₃^-,SO₄^2-and Cl^-),prominent magnetic separation efficacy of 91%,superior reusability with more than 88%of desorption efficiency and 65%of phosphorus uptake maintained after five adsorption-desorption recycles.For La₃-PBC,it was found that the leaching concentrations of La were b etween 0.406 and 0.208 mg.L^-1 within the pH range of 3.0–10.0,while the leaching concentrations of Fe were between 0.108 and 0.024mg.L^-1.For La-Fe BC,it was found that the leaching concentrations of La were between 0.273and 0.057mg.L^-1 within the pH range of 3–11while the leaching concentrations of Fe were between 0.006 and 0.091 mg.L^-1.This demonstrates the leaching of La and Fe from the adsorbents was negligible during the phosphorus adsorption.Adsorption mechanism of phosphorus by adsorbents:La₃-MPBC carried the La speciation of La（OH）₃ and Fe₂O₃.The La specie which was proposed to have supported the achievement of phosphorus binding through the synergistic actions of electrostatic attraction,ligand exchange between the OH group of La₃-MPBC and phosphorus speciation(HPO₄^2-and H₂ PO₄^-),the independence of ionic strength on phosphorus uptake supported the occurrence of inner-sphere complexes on La ₃-MPBC during the adsorption and weak precipitation a strong interaction between La and phosphorus to form LaPO₄.The reduction in the peak corresponding to the Fe-O group as reviewed by the XPS suggests the possible involvement of Fe in the phosphorus uptake onto La₃-MPBC.The La groups(i.e.La（CO₃）₂^-andLa（HCO₃）₂⁺ on La-Fe-BC are believed to enhance the phosphorus-binding owing to a combination of electrostatic attraction and inner-sphere complexation via ligand exchange between these La groups and phosphorus speciation(HPO₄^2-and H₂ PO₄^-).The reduction in the carbonate peak as shown by the FTIR spectra after adsorption suggests that carbonate was involved in the uptake of phosphorus by La-Fe-BC.In addition,the little increase in the peak relating to the Fe-O(i.e.601 cm^-1)and decrease in the peak associated with the–OH as observed in the FTIR spectrum after phosphorus uptake,which showed that Fe and–OH might have contributed to phosphorus uptake by La-Fe-BC.The XPS spectra of both adsorbent s after adsorption showed the peak relating to P 2p,indicating that phosphorus was truly taken by the adsorbent.Bioassay experiment:La-Fe-BC after adsorption of phosphorus was turned into the soil to test its ability to enhance productivity through phosphorus fertilizers for plant growth.It was used in comparison with other phosphorus-laden biochars.These were composed based on individual constituent La-Fe-BC.Thus they included ordinary biochar（BC）,iron fabricated biochar（Fe-BC）,and Lanthanum fabricated biochar（La-BC）.In addition,a mineral fertilizer（single superphosphate-SP）.La-Fe-BC competed favorably with the mineral fertilizer and other biochar fertilizer used.It gave an excellent performance of maize seedlings.It maintained a good soil pH,improving soil macronutrients and micronutrients.The high calcium（Ca）of soil-treated phosphorus laden La-Fe-BC,suggests the formation of calcium-phosphorus complex,which could have resulted in the high uptake of available phosphorus by maize seedlings.The ability of La-Fe-BC to improve the photosynthetic pigment of maize also affirms that lanthanum can improve the photosynthetic capacity of a crop.The plant use efficiency as a factor of the fertilizer source was expressed by the partial factor productivity（PFP）and agronomic efficiency（AE）and recovery rate（RR）.La-Fe-BC gave the best after La-BC.The high performance of La-Fe-BC suggests that it was preventing loss of phosphorus from the soil sorption and enabling more plant usage.In general,La-Fe-BC can be said to have pulled the combined advantages of lanthanum and iron present in the adsorbents to out-perform other adsorbents used in the bioassa y experiment.This indicates that the adsorbents cannot alone be used to depollute phosphorus in water but can in return be used as a phosphorus fertilizer after adsorption.Overall,this study enables the omnidirectional enhancement of tailored adsorbents to the depollution of phosphorus contaminated water and finally reused as a technology for bio-phosphorus fertilizer to enhance agricultural productivity.