Characteristics And Mechanism Of Heavy Metal Ion And Phosphate Adsorption From Aqueous Solutions On Biochar Produced From Caragana Korshinskii And Al Modification

Biochar and chemically modified biochar had received widespread attention of researchers as an environmentally friendly material to remediate contaminated sites with its physico-chemical characteristics in ecological restoration,agriculture and environmental protection.In order to utilize the agriculture and forestry waste biomass resources efficiently in northwest arid area,Caragana Korshinskii were selected to produce biochars at setting different temperature and constant temperature time by oxygen-limited pyrolysis.The preparation data of 15 kinds of Caragana Korshinskii biochar（abbreviation NB）were analyzed,and the physicochemical properties of the NB were characterized by various methods of material analysis,considering with the adsorption capacity of the 15 kinds NB on Pb²⁺,Cd²⁺ and phosphate（abbreviation P）,screening the optimum preparation process,the alternative biochar was obtained.The adsorption characteristics of Pb²⁺,Cd²⁺and P in aqueous solutions were evaluated by conventional adsorption experiments.The adsorption properties of NB on Pb²⁺and Cd²⁺were studied through isothermal adsorption and adsorption kinetics,and the effects experiments of pH,biochar addition amount and ash.The adsorption mechanism of NB on heavy metals was expounded by FTIR（Fourier Transform infrared spectroscopy）.And try to modify NB by Al（aluminum）direct modification method,characterizing the changes of NB before and after Al modified,studying the adsorption characteristics of Al modified NB to Pb²⁺and P,and exploring the adsorption mechanism of P.The results were presented below:（1）The effect of pyrolysis temperature on preparation and physical and chemical properties of biochar higher than the effect of the constant temperature.With the increase of pyrolysis temperature,yield of biochar decreased,the CEC was decreased and then increased,the pH values of biochar raised,BET specific surface area increased,the content of C increased,and the content of O and N decreased.The biochar was produced by different conditions had an initial pH of 6.65⁸.99,CEC of 16.64⁸1.4 cmol·kg-1,specific surface of 13.52¹33.49 m2·g-1.Porous structure was observed by the surface of NB,their surface contained abundant functional groups,such as-OH,-COOH,NH4+ and pyridine et al.the BET specific surface area of NB pyrolyzed at 650 for 3h ℃ reached highest.（2）The adsorption of NB on Pb²⁺ was increased with the increase of initial concentrations and contact time.The isothermal adsorption process of NB on Pb²⁺ can be well simulated by Langmuir model showed that the adsorption of NB on Pb²⁺ was monolayer adsorption.The adsorption kinetics of NB on Pb²⁺ followed pseudo-second order model very well found that the sorption process was physical-chemical process.In combination with other model analysis,the adsorption reaction rate was conducted by intra-particle diffusion,surface adsorption and liquid film diffusion.The adsorption equilibrium of NB to Pb²⁺ was reached in three hours and the maximum adsorption capacity reached 220.94 mg·g-1.With the increase of NB addition amount,the adsorption capacity of Pb²⁺ decreased,the removal rate gradually increasing.In addition,the best addition amount was 3g·L-1 and the optimum pH was between 3⁶.The adsorption mechanism of NB for Pb²⁺ mainly included: electrostatic adsorption（surface negative charge of biochar）,ion（NH4+,-COOH）/ligand（phenolic hydroxyl）exchange and cationic-π（include C=C）.Furthermore,Ash also had positive effect on the mechanism of Pb²⁺ adsorption.（3）The Freundlich model can well describe the isothermal adsorption process of NB to Cd²⁺,which is approximate multilayer adsorption,and the RL value and other fitting parameters showed that the adsorption of Cd²⁺ is beneficial adsorption;the adsorption kinetics of NB to Cd²⁺ was best fitted by the pseudo-second order model,and combined with other models,the adsorption rate was affected by intraparticle diffusion,surface adsorption and liquid film diffusion.The adsorption equilibrium of NB to Cd²⁺ was reached in three hours and the maximum adsorption capacity reached 42.43 mg·g-1.In addition,the best addition amount was 2.2 g·L-1and the optimum pH was between 3⁶.The adsorption mechanism for Cd²⁺ mainly includes: electrostatic adsorption,ligand exchange /ion exchange and cationic-π.（4）Al modified NB had no obvious effect on the adsorption of Pb²⁺.The adsorption capacity of NB to P was very low,however,when the optimum modification ratio of Al modified NB biochar（abbreviation Al-NB）was 0.2:1,then adsorption capacity of P reached the largest,which is 8.35 times of that of unmodified NB.The adsorption kinetics was best fitted by the pseudo-first order model,while the isothermal adsorption was best described by Langmuir isotherms.This indicated that the beneficial adsorption process via monolayer was affected by boundary diffusion.The theory maximum adsorption capacity of Al-NB to P was 19.97 mg·g-1,and the equilibrium time was 24 h.With the increase of Al-NB addition amount,the adsorption capacity of P decreased,the removal rate gradually increasing,and the 2.5g·L-1 was the best addition amount.The optimum pH was 4⁸,when pH=8,the adsorption capacity reached the largest 47.6 mg·g-1.After adsorption P,the pH of the solution was leaned to the neutral range and had a certain buffer effect.Al3+ was mainly loaded on the skeleton of Caragana Korshinskii biochar in the form of hydroxide.The mechanism of P adsorbed by Al-NB mainly includes: electrostatic attraction（mainly positively charged metal oxides surface and the negative phosphate ions,NH4+charge adsorption）,ligand exchange（hydroxyl）,and anion exchange（NO3-）,as well as the inner-sphere surface complex formation process.（5）After retrieving the latest related literatures,comparing the adsorption properties of Pb²⁺and Cd²⁺with selected biochars derived from different materials,it is found that the adsorption capacity of NB in this study was larger,and it has good application prospect and potential.The adsorption capacity of same biochar to Pb²⁺ is generally greater than that of Cd²⁺.The maximum adsorption capacity,adsorption capacity,adsorption strength and adsorption stability of NB to Pb²⁺ were greater than those of Cd²⁺.Under the same conditions,NB was more suitable for removal of Pb²⁺.The adsorption effect of Al modified NB on P was generally higher than that of non-modified biohar and Fe-modified biochar,which was less than Mg or other modified biochar,and has certain application value.In this study,the preparation of Caragana Korshinskii biochar,Al modified and the adsorption characteristics and mechanism of Pb²⁺,Cd²⁺ and P were systematically studied,which provided a theoretical basis for the preparation and functional application of biochar.