Removal Of Orthophosphate From Mariculture Tailwater By Iron-loaded Activated Carbon

Mariculture tailwater refers to aquaculture water discharged from mariculture ponds or farms.Orthophosphate in the mariculture tailwater is one of the potential factors leading to eutrophication.In order to achieve the standard discharge of mariculture tailwater,it is necessary to remove the excessive orthophosphate.At present,the original methods used for orthophosphate removal include biological method,chemical method and adsorption.However,biological method is usually affected by the changes of external environmental factors,and chemical method is easy to cause secondary pollution.Adsorption is an efficient,rapid and environmentally method,and can realize phosphorus resource recovery and utilization.Activated carbon（AC）is a common adsorption material with abundant pore structure,which is widely used in the field of water treatment.However,the adsorption capacity of activated carbon to orthophosphate in water is weak,which can not removal of orthophosphate.Nanomaterials based on iron have high selectivity for orthophosphate adsorption,and their chemical properties are stable.But the powder of nanomaterials is difficult to separate in utilization.In this study,nano-iron compounds were loaded on the surface of activated carbon to obtain iron-loaded activated carbon adsorbent.The adsorption performance and adsorption mechanism of the adsorbent on phosphate in water were explored,and the adsorption and phosphorus removal effect of its application in mariculture tailwater was investigated for providing reference for phosphorus removal.The specific research contents and results are as follows:（1）The nano-ferric hydroxyl oxide（FeOOH）adsorbent was prepared by chemical method,and the nano-ferric trioxide（Fe₂O₃）adsorbent was prepared by hydrothermal method.The adsorption and removal effects of these two iron-based adsorbents on orthophosphate in water were compared.The results showed that the orthophosphate removal rate of nano-FeOOH is higher than that of nano-Fe₂O₃.Meanwhile,the adsorption and phosphorus removal effects of nano-FeOOH loaded with AC were also compared.When the initial concentration of active phosphate in water was 0.50 mg/L,the orthophosphate removal effect of activated carbon loaded with FeOOH（FeOOH@AC）was superior to that of nano-FeOOH and AC.（2）Scanning electron microscopy（SEM）,X-ray diffractometer（XRD）,X-ray photoelectron spectroscopy（XPS）and physical adsorption apparatus（BET）were used to characterize the structure of iron-loaded activated carbon materials.The results showed that both nano-FeOOH and nano-Fe₂O₃ could be uniformly loaded on AC.The material composition and crystalline structure did not change before and after loading.The pore sizes of FeOOH@AC and Fe₂O₃@AC adsorbents are similar,but their specific surface areas are 560.9 and 168.6 m²/g respectively,indicating that the specific surface area of FeOOH@AC is higher than that of Fe₂O₃@AC.（3）Orthophosphate solution prepared by potassium dihydrogen phosphate was used to simulate phosphate in aquaculture tailwater for static adsorption experiment.The results showed that the optimal amount of FeOOH@AC is 20 g/L,and the maximum adsorption capacity of FeOOH@AC is 0.030 mg P/g.The orthophosphate adsorption removal rate of FeOOH@AC increases with the increasing of the initial orthophosphate concentration.When the initial orthophosphate concentration reaches 0.50 mg/L,the phosphate removal rate of FeOOH@AC is no longer increased by further increasing the phosphorus concentration in the treated water.Cl^-,SO₄^2-,HCO₃^-,Ca²⁺and Mg²⁺in water can inhibit the adsorption of FeOOH@AC,among which HCO₃^-has the highest inhibition effect.When the concentration of HCO₃^-in water reaches 0.16g/L,the orthophosphate removal rate of FeOOH@AC is only 20%.（4）From the process of FeOOH@AC phosphate adsorption kinetics and adsorption isotherm fitting analysis,the adsorption process of FeOOH@AC for orthophosphate can be fitted by quasi-second-order kinetic equation,R²>0.999.The adsorption process can be fitted by Langmuir isotherm and Freundlich isotherm,but the fitting effect of Langmuir isotherm is better（R²>0.999）,indicating that the adsorption of orthophosphate on FeOOH@AC is monomoleptic adsorption dominated by chemisorption.（5）An adsorption orthophosphate removal device based on FeOOH@AC adsorbent was constructed to explore the dynamic phosphorus removal effect of adsorbent.The results showed that the orthophosphate removal efficiency of the adsorption phosphorus removal unit increased with the extension of treatment time within 0.5 h,and reached equilibrium after 3 h.With the increase of the influent orthophosphate concentration from 0.10 mg/L to 2.0 mg/L,the time for the adsorbent to reach the adsorption penetration point was shortened from 16 min to 3 min,and the time for the adsorbent to reach and adsorption exhaustion point was shortened from 90 min to 27 min.The faster the influent flow rate increases from 3.8 m L/min to 19 m L/min,the faster the adsorbent reaches the adsorption penetration point from 6 min to 1 min,and the faster the adsorbent reaches and exhausts the point from 25 min to 13 min.When the salinity increases from 5 to 30,the removal rate of orthophosphate decreases from 94.0%to 68.9%.The adsorption saturated FeOOH@AC was regenerated by Na OH and reused,and the results showed that the orthophosphate removal rate of FeOOH@AC could still reach 90%after 5 times using,indicating that FeOOH@AC had a good stability.（6）The actual mariculture tailwater from a fish farm in Dalian was selected as the treatment object.The results showed that the concentration of orthophosphate in mariculture tailwater decreased from 0.652 mg/L to 0.028 mg/L,and the removal rate reached more than 95%,indicating an obvious orthophosphate removal.After treatment,the concentration of orthophosphate in mariculture tailwater decreased to less than 0.05 mg/L,which has met the requirements of mariculture tailwater discharge standard.