| Water eutrophication is of great attention worldwide,since excessive nutrients lead to rapid propagation of algae in the water.For example,a large-scale outbreak of cyanobacteria occurs every year in the Taihu Lake basin,negatively affecting the development of surrounding cities.Phosphate is one of the key factors for water eutrophication.To overcome these challenges,using biochar prepared by blue algae to remove phosphate from contaminated wastewater can not only alleviate the eutrophication problem in water bodies but also solve the problem of blue-green algae accumulation.Therefore,the present study investigated the potential of polymeric ferric sulfate（PFS）and polymeric aluminum chloride（PAC）modified blue algae biochar under certain conditions for phosphate removal.the characterizations（e.g.,surface morphology,crystal structure,and physicochemical properties）of modified blue algae biochar were determined to reveal methods analyzed the phosphate adsorption mechanism.As a soil modifier and phosphate fertilizer,the effect of phosphate loaded biochar on soil characteristics and maize plants was investigated.The results of this study will provide a theoretical basis for the resource utilization of blue algae,the reduction of lake eutrophication,and the recycling and utilization of biochar resources.The main research results are as follows:（1）The optimal preparation conditions for blue-green algae biochar were determined via response surface method（RSM）optimization using PFS assisted dehydration of salvaged blue-green algae as raw materials.The results showed that the dosage of PFS was 458 mg·L-1,the reaction temperature was 433℃,and the mass ratio of biochar precursor to water vapor was 1:11.Various techniques such as scanning electron microscopy,X-ray diffraction,Fourier transform infrared spectroscopy,and Raman spectroscopy were used to characterize and analyze biochar.Compared with the original biochar（F0H11-433）,it was observed that iron oxide was loaded onto the iron salt modified blue algae biochar（F458H11-433）,and the zero charge point（p Hpzc）increased from 4.41 to 6.19,the degree of disorder and defect of biochar has been improved.In addition,the phosphate adsorption capacity of the physically mixed F0H11-433+FH11-433（FH11-433 was prepared using PFS）is not as good as that of F458H11-433,indicating that changes in the p Hpzc of biochar,the extent of defects,the increase in pore structure and functional groups,and the dispersion and loading of biochar on iron oxides caused by the co pyrolysis of PFS and blue algae have a synergistic enhancement effect on phosphate adsorption.The adsorption process data are following the pseudo-second order model and Langmuir model,and the saturated adsorption capacity is 31.97 mg·g-1,which is 2.63 times the adsorption capacity of F0H11-433.From the characterization before and after adsorption,it can be seen that the adsorption mechanism of F458H11-433 for phosphate mainly involves electrostatic attraction and ligand exchange.The removal performance of F458H11-433 on phosphate in actual lake water was investigated.The results showed that the residual amount of phosphate in the effluent of F458H11-433 was 0.025 mg·L-1at the dosage of 0.3 g·L-1;After five cycles,the phosphate removal rate of F458H11-433 can still reach 75.78%,indicating that F458H11-433 has renewable characteristics;The dynamic adsorption experiment shows that F458H11-433 has good dynamic adsorption capacity for phosphate in actual lake water.The above data also proves the feasibility of F458H11-433 as an artificial wetland filler for lake water treatment or as an adsorbent for advanced treatment of phosphate in waste（sewage）water.（2）PAC is also used in actual blue algae dehydration,so aluminum salt modified blue algae biochar was prepared using salvaged blue algae after PAC assisted dehydration as raw material.RSM optimized the preparation conditions.The results showed that the dosage of PAC was 500 mg·L-1,the carbonization temperature was 500℃,and the mass ratio of biochar precursor to KOH was 1:1.5.The characterization results show that compared with the original biochar(P0K1.5-500),the main crystalline components areα-Al2O3 andγ-Al OOH aluminum oxide successfully loaded onto the surface of aluminum salt modified biochar（P500K1.5-500）.In addition,compared to P0K1.5-500+PK1.5-500（PK1.5-500 was prepared using PAC）with the physically mixed,P500K1.5-500 still has significant advantages in phosphate adsorption due to the uniform dispersion of aluminum oxides on the surface,increased zero charge point（p Hpzc）,increased disorder and defect degree of carbon materials,and an increase in overall pore structure and functional groups.The interaction between PAC and blue algae during co pyrolysis and activation has a synergistic strengthening effect on subsequent phosphate removal.The adsorption experiment results showed that the adsorption process of P500K1.5-500 for phosphate follows the pseudo-second order model and Langmuir model,with a saturated adsorption capacity of 58.72 mg·g-1,significantly higher than F458H11-433.The characterization experiments of the materials before and after comprehensive adsorption show that the phosphate removal mechanism of P500K1.5-500 mainly includes electrostatic attraction,ion exchange,chemical precipitation,and ligand exchange.Compared to F458H11-433,the higher p Hpzc（7.25）and chemical precipitation phosphate removal mechanism of P500K1.5-500are the main reasons for its strong phosphate removal ability.Therefore,when the dosage of P500K1.5-500 is 0.05 g·L-1,the residual amount of phosphate in the actual lake water containing phosphate can reach 0.022 mgg·L-1 after being treated with P500K1.5-500.At the same time,P500K1.5-500 has good dynamic adsorption capacity and regenerative cycling adsorption capacity for phosphate.The above data also indicates that P500K1.5-500 has the potential to be used as artificial wetland filler for lake water treatment or as an adsorbent for advanced treatment of phosphate in waste（sewage）water.（3）The application ability of phosphate loaded blue algae biochar in soil improvement was investigated.Compared with the control group,the soil p H value of the phosphorus loaded biochar treatment group increased,the cation exchange capacity（CEC）content increased by 20.13%～28.38%,the total phosphorus content increased by 14.61%～27.15%,the available phosphorus content significantly increased（583.33%～935.03%）,and the organic matter content increased by 75.56%～91.69%.Among them,phosphate loaded iron salt modified blue algae biochar has a better effect on improving the effective phosphorus content in soil compared to phosphorus loaded aluminum salt modified blue algae biochar.The main reason is that the available phosphorus content in phosphorus loaded iron salt modified blue algae biochar is higher than that in phosphorus loaded aluminum salt modified biochar at the same phosphate adsorption capacity,which is related to the adsorption mechanism of both for phosphorus.The growth and development of corn plants in the phosphate-loaded biochar treatment group were significantly improved.The fresh weight of aboveground parts of corn plants increased by 79.88%～104.27%,the fresh weight of underground roots increased by67.07%～95.78%,the plant height increased by 66.76%～93.99%,the stem diameter increased by 30.00%～63.33%,the phosphate（P）content of aboveground parts increased by29.70%～54.91%,and the P content of underground roots increased by 28.20%～69.27%.The experimental results indicate that blue algae biochar after phosphate absorption can effectively improve soil and promote plant growth as a soil modifier. |
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