Adsorption Of Nitrate From Water By Modified Biochar

Due to the unreasonable discharge of nitrate nitrogen?NO₃-N?-containing wastewater,the problem of nitrogen pollution in various types of water bodies has become increasingly prominent.At present,rare earth elements are widely used due to their optical and electromagnetic properties,but the alkali roasting-dilute nitric acid leaching method used in the separation of rare earth heavy metals often results in the production of wastewater which contains NO₃-N.In order to treat NO₃-N in water bodies?especially industrial wastewater represented by rare earth wastewater?,this study uses commercial products such as coconut shell biochar?CSHB?,corn straw biochar?CSTB?and pine sawdust biochar?SDB?.The three kinds of raw commercial biochars used as the carbon-based materials were respectively modified by Fe Cl₃,Al Cl₃,Fe Cl₃and Al Cl₃,and the nine modified biochars were respectively used to adsorb NO₃-N from simulated wastewater?The experiment used two concentrations of NS,NS1 and NS2,respectively?and rare earth wastewater?pretreated,recorded as REW?,from which the best two kinds of modified biochars were screened and applied to remove NO₃-N from NS1,NS2 and REW.The effects of two screened modified biochars on the adsorption of NO₃-N were investigated by single factor adsorption tests such as adsorption time,dosage of adsorbent,solution p H and solution temperature.The experimental results were fitted by kinetics,isothermal and thermodynamic models,and combining with the characterization of biochar before and after modification and NO₃-N adsorption.The mechanisms of NO₃-N adsorption by screening modified biochar in NS and REW are discussed.The main conclusions are as follows:?1?Metal modification such as iron modification,aluminum modification and iron-aluminum modification can greatly improve the adsorption capacity of raw biochar for NO₃-N,iron-aluminum modified coconut shell biochar?Fe-Al/CSHB?and aluminum Modified straw biochar?Al/CSTB?both showed good adsorption performance for NO₃-N in NS1,NS2 and REW compared with other modified biochars.Combined with SEM and BET characterization results,it is found that the particles of Fe-Al/CSHB and Al/CSTB are more closely combined than the original biochars,but the porosity and specific surface area are reduced,which is related to the loading of iron or aluminum-containing substances.CSHB,Fe-Al/CSHB and Al/CSTB all have a large number of mesoporous structures,and CSTB has abundant macroporous structure.XRD results show that Fe-Al/CSHB and Al/CSTB have iron or aluminum particles on the surface.Combined with FTIR and XPS characterization,it was found that the iron element of Fe-Al/CSHB existed in the form of Fe²⁺,Fe₂O₃and Fe OOH,and the aluminum element of Fe-Al/CSHB and Al/CSTB both existed in the form of Al₂O₃.?2?The adsorption processes of NO₃-N in NS1,NS2 and REW by Fe-Al/CSHB and Al/CSTB can reach equilibrium within 20 min.The quasi-second-kinetic model can better describe the adsorption process of NO₃-N in all cases.Fe-Al/CSHB has the highest removal rate of NO₃-N in REW when the dosage of adsorbent is sufficient;The isothermal adsorption of NO₃-N by Fe-Al/CSHB and Al/CSTB at different temperatures are both more in line with Freundlich isothermal adsorption model.The adsorption of NO₃-N by Fe-Al/CSHB and Al/CSTB are both spontaneous endothermic reactions with entropy increasing.It is beneficial to the adsorption of NO₃-N in NS1,NS2 and REW by Fe-Al/CSHB and Al/CSTB.The adsorption amount of NO₃-N in REW decreases with the increase of temperature by Fe-Al/CSHB,while the adsorption amount of NO₃-N by Al/CSTB in REW increased with the increase of temperature.Coexisting anions inhibited NO₃-N by Fe-Al/CSHB and Al/CSTB,and the larger the n?coexisting ions/NO₃^-?,the more obvious the inhibition effect,Fe-Al/CSHB is less affected by the coexisting anions than the Al/CSTB.Ca²⁺promoted Fe-Al/CSHB adsorption of NO₃-N,and the larger the n(Ca²⁺/NO₃^-),the more obvious the promotion effect,but Ca²⁺did not affect the adsorption of NO₃-N by Al/CSTB.?3?There are two main mechanisms for the removal of NO₃-N in NS by Fe-Al/CSHB:?1?Ligand Exchange between M-O,-OH in Fe₂O₃,Fe OOH and Al₂O₃and N-O in NO₃-N;?2?Fe²⁺and NO₃-N undergo a redox reaction.The removal mechanism of NO₃-N in NS by Al/CSTB is the ligand exchange between Al-O in Al₂O₃and N-O in NO₃-N.There are three main mechanisms for the removal of NO₃-N in REW by Fe-Al/CSHB:?1?Fe₂O₃,Fe OOH and Ca²⁺form positively charged iron-calcium complexes,and NO₃-N is removed by electrostatic attraction;?2?Fe²⁺and NO₃-N undergo redox reaction;?3?ligand exchange occurs between Al-O in Al₂O₃and NO in NO₃-N.The removal mechanism of NO₃-N in REW by Al/CSTB is the ligand exchange between Al-O in Al₂O₃and N-O in NO₃-N.?4?In all cases,the optimal conditions for NO₃-N removal in REW are:using Fe-Al/CSHB,with adsorbent dosage of 5.3 mg/L,solution p H of 1.0,and solution temperature of 35°C.At this time,the removal rate of NO₃-N in REW was 80.79%.