Adsorption Of Heavy Metals From Aqueous Using Stabilized Nanoscale Zero-valent Iron Biochar

With the acceleration of industrialization and urbanization,the problems of depletion of social resources and environmental pollution have become increasingly prominent.How to use waste resources to control environmental pollution effectively has become a hot issue to all countries.As a large agricultural country,China has abundant of crop biomass resources.However,the use of incineration to treat agricultural waste not only waste resources,but also pollute environment.The addition of transition metal ions in the pyrolysis process can improves the quality of biofuels,at the same time produces composite materials of biochar and metal nano-ions,which has great potential for environmental restoration.Nano-zero-valent iron is widely used in the treatment of heavy metal pollutants due to its small particle size,large specific surface area and strong reactivity.However,due to the instability of nano-zero-valent iron,it is easy to react with oxygen and water.At the same time,due to the existence of gravity,magnetic force and van der Waals force,agglomeration is very easy to occur;the disadvantage of difficult separation and recovery makes the reactivity and the recycling rate reduced,which inhibits its application in industry.In order to solve such problems,the preparation of stabilized and easily recyclable nano-zero-valent iron adsorption materials for practical sewage treatment has become an urgent research topic.In order to solve the above problems,the main research work and achievements are as follows:?1?In order to improve the stability of nano-zero-valent iron and implement the concept of turning waste into treasure,this study use agricultural waste--corn stalk.We synthesized corn stalk biochar loaded with nano-zero-valent iron?B-nZVI?in one step by high-temperature pyrolysis corn stover powder,which impregnation of Fe³⁺,under anaerobic conditions.At the same time,corn stover biochar?B?was prepared.Because powder adsorbent materials have the disadvantages of being difficult to recycle and reuse in industrial practice,this study uses bio-embedding method to embed B-nZVI in alginic acid which has great biocompatibility and biodegradability.In this way,sodium alginate/nano-zero-valent iron biochar?SA/B-nZVI?was prepared.At the same time,a pure sodium alginate pellet?SA?was prepared.The materials were characterized by SEM,FTIR and XRD.It was found that nano-zero-valent iron was successfully loaded on corn stover biochar,which inhibited the shortcomings of easy agglomeration.The particle size of nano-zero-valent iron was 27.60 nm.And it is evenly embedded in the pellet.?2?Four materials were used to study the adsorption of Pb²⁺,Cu²⁺and Cd²⁺in aqueous solution,and to select the best adsorption conditions.Studies have shown that the adsorption capacity of SA/B-nZVI pellets on three heavy metals was significantly higher than the other three materials.Compared with B-nZVI,due to the increase of mass transfer resistance and contact area,SA/B-nZVI has a longer removal time.At the same time,because sodium alginate is rich in hydroxyl groups,carboxyl groups and other groups can chelate with heavy metals,the amount of heavy metal adsorption will be more than B-nZVI.Under optimal adsorption conditions,for Pb²⁺,the maximum adsorption capacities of B,B-nZVI,SA,SA/B-nZVI were:50.21mg/g,103.43 mg/g,221.80 mg/g,349.45 mg/g.For Cu²⁺,the maximum adsorption capacities of B,B-nZVI,SA,SA/B-nZVI were:20.22 mg/g,60.46 mg/g,113.64 mg/g,and 146.83 mg/g,respectively.For Cd²⁺,the maximum adsorption capacities of B,B-nZVI,SA,and SA/B-nZVI were 10.08 mg/g,48.28 mg/g,72.40 mg/g,and 105.56mg/g,respectively.?3?Kinetic fitting of B-nZVI and SA/B-nZVI beads showed that the two composites are fit quasi-secondary kinetic adsorption model.The isotherm study fitting of B-nZVI and SA/B-nZVI beads showed that the composites fit Langmuir adsorption.Calculated by Langmuir model,the maximum adsorption capacity of B-nZVI for Pb²⁺,Cu²⁺and Cd²⁺was 107.64 mg/g,66.27 mg/g and 51.84 mg/g;the maximum adsorption capacities of SA/B-nZVI for Pb²⁺,Cu²⁺and Cd²⁺were 350.88mg/g,153.14 mg/g and 108.69 mg/g,respectively.The thermodynamic study of the SA/B-nZVI beads shows that the adsorption process is an endothermic process.?4?The competive adsorption studies on the SA/B-nZVI beads shown that the presence of Pb²⁺has the great effect on the adsorption of Cu²⁺and Cd²⁺.The SA/B-nZVI prepared by the embedding method reduces the environmental risk of losing the nano material in water.Compared with other materials,it is easy to separate from water and improved the quality of effluent water.After recycling of 5 times,the removal rates of Pb²⁺,Cu²⁺and Cd²⁺were 87%,79%and 73%,respectively.?5?The SA/B-nZVI pellets were used to simulate constructed wetlands to explore their feasibility in practical heavy metal contamination removal applications.The results shown that the addition of SA/B-nZVI beads in the simulated constructed wetland could increase the removal rate of Pb²⁺,Cu²⁺and Cd²⁺,and the removal rate increased with the addition of SA/B-nZVI beads increased.Recycled three times,the removal rates of Pb²⁺,Cu²⁺and Cd²⁺in simulated wetland were 94.24%,86.36%and83.12%,respectively.Studies have shown that the use of SA/B-nZVI beads for the removal of heavy metal pollution in constructed wetlands is feasible.