Study On Competitive Adsorption Characteristics And Mechanisms Of Heavy Metals By Biochar Derived From Water Hyacinth

In the process of mining and smelting,it is easy to cause serious heavy metal pollution and Cu,Pb,Cd and Zn are four typical heavy metal pollutants commonly seen in this process.These heavy metals have strong enrichment properties in the environment and are difficult to degrade.When these havy metals enter the ecological environment will cause great harm to human health and the ecosystem.Currently,there are many treatment methods for heavy metals,including chemical precipitation,ion exchange,membrane technology,and so on,but these methods have some problems such as high cost and unsatisfactory effect.Compared with these traditional methods,adsorption is considered to be a promising treatment method for heavy metals due to its cost and efficiency.The key factor in adsorption method is adsorption material.At present,the most widely used adsorption material is biochar which prepared from biomass material.The raw materials of biochar come from a wide range of sources,so it is particularly important to study the selection of raw materials to prepare biochar.Water hyacinth?Eichhornia crassipes?is a kind of floating perennial herbaceous plants,native to South America Amazon river basin.Water hyacinth has a strong ability to reproduce,it can be sexual and asexual reproduction at the same time,and has a strong ability to adapt to the environment,and overbreeding can cause serious harm to water quality.However,water hyacinth has strong tolerance and adsorption ability to heavy metals,nutrient salts such as N,P and other pollutants,so it has certain application value in environmental pollution control.Therefore,the use of mature water hyacinth plants to prepare adsorption materials,and the application of the removal of pollutants in the water,not only can remove pollutants in the water,but also realize the resource utilization of mature water hyacinth,reduce the cost of adsorption materials.In this study,stem powder,root powder and water hyacinth biochar were prepared from mature water hyacinth plants,which were used in the adsorption of heavy metals.This study has a good theoretical guiding significance for the resource utilization of water hyacinth,the competitive adsorption of multi-component heavy metals,the interaction between biochar preparation conditions and the remediation and treatment of heavy metal pollution from tailings.The main research conclusions are as follows:?1?Using water hyacinth as raw material,preparation for three types of adsorption material,stem powder,root powder and biochar.Three adsorption materials were applied to the adsorption of Cu²⁺,Pb²⁺,Cd²⁺and Zn²⁺in aqueous solution.The effects of adsorbent dosage,reaction time and initial concentration of heavy metal ions on adsorption were investigated.Langmuir and Freundlich isothermal equations were used to fit the adsorption results,by fitting,the maximum adsorption capacity of Cu²⁺by stem powder,root powder and biochar was 131.75 mg/g,134.91 mg/g and 177.66 mg/g,respectively;the maximum adsorption capacity of Pb²⁺was 106.86 mg/g,113.44 mg/g and 195.24 mg/g,respectively;the maximum adsorption capacity of Cd²⁺was 84.79 mg/g,103.57 mg/g and 142.59 mg/g,respectively;the maximum adsorption capacity of Zn²⁺was 98.95 mg/g,129.99 mg/g and146.14 mg/g,respectively.?2?Six binary systems,four ternary systems and one quaternary system were composed of Cu²⁺,Pb²⁺,Cd²⁺and Zn²⁺,using the water hyacinth biochar as adsorbent,to study for competitive adsorption at the same concentration.The order of adsorption capacity of water hyacinth biochar to binary system was obtained as follows:Pb²⁺>Cu²⁺,Cu²⁺>Cd²⁺,Cu²⁺>Zn²⁺,Pb²⁺>Cd²⁺,Pb²⁺>Zn²⁺and Cd²⁺>Zn²⁺,the order of adsorption capacity in ternary system was:Cu²⁺>Cd²⁺>Pb²⁺,Zn²⁺>Pb²⁺>Cu²⁺,Zn²⁺>Cu²⁺>Cd²⁺and Pb²⁺>Zn²⁺>Cd²⁺,and the order of adsorption capacity in quaternary system was:Zn²⁺>Pb²⁺>Cd²⁺>Cu²⁺.The results showed that the adsorption capacity of water hyacinth biochar to heavy metal ions in the binary system was the same as that of the single component,while the ternary and quaternary system different from adsorption rule of single component system.Combined with characterization of biochar and the different characteristics of heavy metal ions,the adsorption mechanism is analyzed.The analysis shows that in the binary system,the inhibition of Cu²⁺,Cd²⁺and Zn²⁺on Pb²⁺adsorption by biochar was stronger than that of Pb²⁺on Cu²⁺,Cd²⁺and Zn²⁺,and in ternary and quarternay systems,the main function of heavy metal adsorption is the complexation between functional groups and heavy metal ions,precipitation and ion exchange have little effect on the multicomponent system.?3?Response surface method was used to optimize the preparation conditions of biochar.Four different optimal combination conditions for biochar with adsorption of Cu²⁺,Pb²⁺,Cd²⁺and Zn²⁺were obtained.The optimal preparation conditions of Cu²⁺adsorbed biochar were:425.271?,3.090 h,19.649?/min;the optimal preparation conditions for Pb²⁺adsorbed biochar:433.827?,2.654 h,19.961?/min;the optimal preparation conditions of Cd²⁺adsorbed biochar were 392.997?,2.424 h and 15.559?/min;the optimal preparation conditions for adsorption of Zn²⁺were 421.974?,2.192 h and 15.883?/min.The maximum adsorption capacity of heavy metal ions adsorbed by 4 kinds of biochar by Langmuir model are:Cu²⁺?210.56 mg/g?,Pb²⁺?251.39 mg/g?,Cd²⁺?186.18 mg/g?and Zn²⁺?223.32 mg/g?.The adsorption capacities are better than that of unoptimized biochar.?4?A phosphate-copper tailings-water hyacinth adsorbent system was constructed to repair and treat heavy metal pollution caused by tailings.Firstly,the phosphate leachate modified copper tailings system was established,which combined with the passivation effect of phosphate on heavy metals,reduced the biological availability of heavy metals in tailings,thus reduced the absorption of heavy metal ions by plants in tailings,and reduced the ecological harm.Then water hyacinth adsorption materials were applied to adsorb the leachate,and the Cu²⁺and Zn²⁺reached the second-level and first-level discharge standards in the comprehensive sewage discharge standard?GB8978-1996?,respectively.Although the removal of Cd²⁺in the system did not reach the first-level discharge standard,the removal rate of Cd²⁺reached 94.94%.Through the establishment of phosphate-modified copper tailings-water hyacinth adsorption materials system,the method of chemical passivation-adsorption was combined.The heavy metals in copper tailings were removed by passivation,leaching and adsorption.It reduces the biological availability of copper tailings and the content of heavy metals in leachate,thus reducing the harm to the ecological environment and providing an effective way for the adsorption of soil heavy metal pollution caused by tailings.