Study On Highly Efficient Phosphorus Recovery From Solid Wastes Of Recirculating Aquaculture By Adsorption

The recirculating aquaculture system will produce a large amount of solid waste during the farming process,which has the characteristics of complex composition,low phosphorus content per unit mass,and high total emission,becoming one of the inevitable challenges in the sustainable development of aquaculture.The removal and recovery of phosphorus in solid waste can not only avoid water eutrophication but also alleviate the depletion of phosphorus resources.To evaluate the feasibility and necessity of phosphorus recovery from the pond recirculating aquaculture system solid waste,this paper firstly carried out phosphorus species classification and content measurement in solid waste,the effect study of p H adjustment and ultrasonic assistance on phosphorus release,and the roles determination of different phosphorus species in the release process.Secondly,to achieve the selective removal of released phosphorus,the ZrO₂@Fe₃O₄was chosen due to its acid-resistance and easy-to-separate.The adsorption performance and adsorption mechanism were investigated.The feasibility and repeatability of simultaneous phosphorus release and adsorption were evaluate.Thirdly,according to the enhanced phosphorus adsorption phenomenon of bimetallic composites,the La-Zr@Fe₃O₄was synthesized to explore the effect and regulation mechanism of La doping on phosphorus adsorption performance.Next,considering the enhancement of the unique structure of the metal-organic framework on phosphorus adsorption,UiO-66@Fe₃O₄was prepared to investigate the influence of Fe₃O₄on the structure of UiO-66,and to clarify the relationship between structural changes and phosphorus adsorption performance.Finally,based on the characteristics of the magnetic adsorbent,a continuous reactor was constructed to track the phosphorus removal efficiency of long-term continuous operation.Conditions of phosphorus recovery from desorption solution were studied,achieving a closed loop of phosphorus release-adsorption-recovery,proposing a new treatment scheme and research basis for recirculating aquaculture system solid waste treatment.The main results are as follows:（1）Phosphorus composition of solid waste and removal by adsorption:the total phosphorus content of solid waste from the pond recirculating is 4.28 mg/g,which mainly exists as inorganic apatite and slowly releases phosphate to the water body over 0.5 mg/L.Since AP tends to dissolve under acidic conditions,acidification is beneficial to enhance phosphorus release from aquaculture solid waste.Under the auxiliary action of the ultrasonic cavity effect and thermal effect,NAIP and OP_swill be dissolved and transformed,enhancing the phosphorus release and reaching the highest phosphorus release efficiency of 49%.Mixing ZrO₂@Fe₃O₄with acidified solid waste can achieve simultaneous phosphorus release and adsorption,and the removal efficiency is affected by the mass ratio of solid waste to the adsorbent.When the ratio of solid waste to adsorbent is 3:1,the phosphorus removal and desorption efficiency after multiple adsorption-desorption cycles can still maintain a good level of more than 90%and 113%,showing its excellent reusability.（2）Phosphorus adsorption by bimetallic adsorbent La-Zr@Fe₃O₄:in order to improve the adsorption performance of ZrO₂@Fe₃O₄,the bimetallic modified magnetic adsorbents La-Zr@Fe₃O₄is prepared by doping La metal to ZrO₂@Fe₃O₄with co-precipitation method.Compared with ZrO₂@Fe₃O₄,the strong affinity inherited from the parent metal La and the synergistic effect between La and Zr metal increase by 77%of the adsorption capacity to 49.1 mg/g.However,the introduction of La will reduce the desorption efficiency of the adsorbent due to the strong binding of La PO₄.During the release of phosphorus from solid waste acidification treatment,La-Zr@Fe₃O₄can enhance the phosphorus release efficiency by 10%.After five adsorption-desorption cycles,the removal and desorption efficiency are still higher than 90%,showing good reusability.The adsorption kinetics and isotherm of La-Zr@Fe₃O₄are well described by the pseudo-second-order model and the Freundlich isotherm model,respectively,indicating the heterogeneous distribution of adsorption sites and the phosphate multilayer adsorption.The phosphate adsorption mechanism for La-Zr@Fe₃O₄was the inner-sphere complexation via the ligand exchange between OH^-and phosphate,which would not be affected by the coexisting anions.（3）Phosphorus adsorption by metal-organic framework UiO-66@Fe₃O₄:in order to further improve the adsorption performance of ZrO₂@Fe₃O₄,the terephthalic acid organic ligands are introduced as linkers to form the Zr-based magnetic metal-organic framework UiO-66@Fe₃O₄by hydrothermal method.The unique structural characteristics of UiO-66@Fe₃O₄enable its excellent adsorption performance.The maximum adsorption capacity reaches 136.54 mg/g,while the desorption efficiency is over 95%.The addition of Fe triggers the change of the coordination numbers of Zr metal,forming more structural defects and improving the adsorption capacity,resulting in the adsorption performance of UiO-66@Fe₃O₄remaining unchanged even with the increase of Fe₃O₄content.Besides,the addition of Fe₃O₄makes the UiO-66@Fe₃O₄kinetics and adsorption isotherm model more consistent with the Elovich kinetics model and the Freundlich isotherm adsorption model,which means that the adsorption sites and adsorption modes are heterogeneous distribution and multi-layer adsorption mode.When treating neutral low-concentration actual wastewater with 0.2 g/L UiO-66@Fe₃O₄,the phosphate removal and desorption efficiency could maintain at about 80%and 95%after multiple cycles.The phosphate adsorption mechanism on UiO-66@Fe₃O₄was the ligand exchange between OH^-and phosphate and linker exchange.Although the linker exchange could improve the adsorption capacity,it will destroy the crystal structure,which is not conducive to long-term application.（4）Phosphorus removal and recovery by continuous reactor:the construction of the continuous reactor is based on the ZrO₂@Fe₃O₄magnetic adsorbent.The kinetics of treating 1 mg/L low phosphorus concentration wastewater at different adsorbent dosages are studied by static experiments.For the subsequent continuous experiments,the adsorbent dosage and hydraulic retention time are determined to be 1 g/L and 1 h,respectively.The phosphorus removal performance of the continuous reactor is closely related to the initial concentration of phosphate,p H,etc.Under the situation of stable influent water quality parameters,with the help of ultrasonic to alleviate the agglomeration phenomenon of ZrO₂@Fe₃O₄,the continuous reactor can stably reduce the phosphate below 0.05 mg/L during more than 20 cycles.Besides,the phosphate can be concentrated by reusing the desorption solution multiple times until reaching the requirement of chemical precipitation.The hydroxyapatite（HAP）precipitation in the desorption solution can be realized by adjusting the p H and Ca∶P molar ratio properly,closing the loop of phosphate recovery,which can reduce the cost by decreasing the loss of the desorption solution.