| Regulations on phosphorus(P)discharge is becoming more stringent since P is a key contributor for eutrophication,and advanced treatment and recovery of P have been a hot topic for decades.Among various treatment methods,adsorption is one of the promising ways to address the P issue due to its capacity to reduce P to ultra-low level,ease of operation and maintenance,and potential for phosphate recovery from wastewater.In recent years,The use of metal(e.g.,lanthanum,iron,aluminum,manganese,titanium,zirconium and cerium)(hydr)oxides as adsorbents to remove phosphate from contaminated waters has gained increasing attention.Particularly,metal(hydr)oxide-based hybrid adsorbents combine the specific affinity of metal(hydr)oxides toward phosphate as well as the excellent hydraulic and mechanic performance of the hosts,exhibiting great potential in scale-up water treatment.For instance,polystyrene-based nano metal-oxide composites have been successfully applied in scaled-up water decontamination ffor phosphate.Calcium ion(Ca2+),a ubiquitous cation in P-containing wastewater,can influence the adsorption of phosphate on metal oxides such as iron oxides,lanthanum oxides,zirconium oxides and titanium dioxides.However,the effect of Ca2+ on the long-term adorption and regeneration of the nanocomposite remains unclear.In this study,cyclic adsorption-desorption experiments were carried out to evaluate the effect of Ca2+ on phosphate adsorption onto polymer-supported Fe(Ⅲ)nanocomposite HFO@201 as well as polymersupported La(Ⅲ)nanocomposite HLO@201.For HFO@201,an unexpectedly enhanced phosphate removal was observed in the presence of Ca2+ during 20 cyclic runs,which is distinct from its adverse effect on phosphate capture by granular ferric(Hydr)oxide(GFH).For example,the presence of 160 mg/L Ca2+ increased the P capacity from～7 mg/g(Ca2+ free)to～20 mg/g at the 6th cycle.The enhanced phosphate removal was mainly realized via the Ca-P coprecipitation inside the networking pores of HFO@201 as well as the possible formation of the multiple Fe-P-Ca-P complex.Theoretical calculation suggested that the local pH inside the polymeric networking pores is much higher than the surrounding solution during P adsorption,which is favorable for the formation of amorphous calcium phosphate(ACP)and hydroxyapatite(HAP)inside.STEM-XEDS spectra indicated that coprecipitation did not occur on the surface of loaded nano-HFO,greatly mitigating its adverse effect on P adsorption by nano-HFO.Fixed-bed column adsorption showed that the presence of Ca2+ increased the effective treatable volume of HFO@201 toward P-containing influents from 700 BV(Ca2+ free)to 1200 BV with insignificant capacity loss during 3 cycles.For HLO@201,the 20 cyclic runs showed that the presence of Ca2+ enhanced phosphate removal of HLO@201 initially.However,it turned out to be negative as the operation continued.For example,with the addition of 80 mg/L Ca2+,P capacity was enhanced from～16 mg/g(Ca2+free)to～25 mg/g at the 4th cycle,while it dropped down to～13 mg/g at the 14th cycle.La-P-Ca ternary complexes and La-P-Ca-P quaternary complexes were possibly formed on the surface of HLO during adsoption,which were beneficial to the removal of phosphate on HLO@201.On the other side,HAP precipitation and nonrenewable LaPO4·xH2O were formed in HLO@201,resulting in more inaccessible sites of HLO as well as the lower regeneration of HLO@201.STEM-XEDS elemental distribution indicated that Ca-P precipitation coexisted with LaPO4·xH2O,hindering the regeneration of HLO.Fortunately,two-step acid-base regeneration approach could refresh the phosphate capacity of HLO@201.This study could provide important guidance for application of HFO@201 and HLO@201 in advanced P removal in practical application.Besides,it provided new insights into the effect of environmental factors on similar nanocomposites for advanced water treatment. |
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