| Eichhornia crassipes,also known as water hyacinth,is one of the most threatened invasive plant species in the world.Its rapid reproduction ability and strong adaptability make it difficult to be cured.In China,mechanical harvesting and artificial fishing are the main methods to prevent the continuous spread and invasion of Eichhornia crassipes.As a result,a large amount of discarded biomass is produced every year.However,the huge amount of waste of Eichhornia crassipes has not been utilized in high value,and most of them are incinerated,buried or discarded on the spot,causing secondary pollution to the ecological environment.The other hand,this kind of waste biomass with abundant sources and low cost is an ideal precursor material for biochar.In recent years,with the rapid development of human industry,heavy metal pollution has become increasingly serious,especially mercury with the discharge of industrial waste gas and wastewater into the atmosphere and water,and eventually diffuse and accumulate in the soil and sediment.Inorganic mercury has been proved to be a major toxic substance,and under biological action,it will be transformed into more toxic methylmercury,enriched and transmitted along the food chain,seriously endangering human life.By preparing the waste biomass of Eichhornia crassipes into magnetic recyclable activated biochar,it can be used as an excellent mercury element adsorbent and detection agent.After washing,drying,grinding and sieving,the waste biomass of Eichhornia crassipes was impregnation into FeSO4 solution of 0.1 mol/L and FeCl3 solution of 0.1 mol/L,respectively,and then pyrolyzed with the original material in muffle furnace at 450℃,600℃,750℃ and 900℃ for 2 h.A total of 8 kinds of magnetically active biochars and 4 kinds of normal biochars as control group were prepared.The structure of the prepared biochar was characterized by SEM,EDS,BET,XRD,FTIR and XPS,and the performance of biochar to adsorb different concentrations of total mercury(1000 μg/L,500 μg/L,100 μg/L,10 μg/L and 2 μg/L)and methylmercury(10 μg/L,1 μg/L and 0.1 μg/L)in the water phase was studied.The research results show that the prepared biochars are mesoporous material,the pore size is mainly distributed in 2-50 nm,and the specific surface area of pyrolyzed biochars at 600℃ is the largest,of which the specific surface area of BC600-3 is 120.48 m2/g.Pyrolysed biochars at 750℃ has the highest oxygen-containing functional group content.Compared with non-activated biochars,the oxygen-containing functional groupcontent of activated biochars decreases,but the load of active sites of S and Cl can effectively make up for the loss of this part of adsorption sites,and the load of active sites related to S and Cl is the highest at 600℃.The surface of active biochars were loaded with magnetic iron oxides,which increased with the increase of temperature and reached the highest content at 750℃.The magnetic components of FeSO4-activated biochars included Fe3O4 and Fe2O3,while the magnetic components of FeCl3-activated biochar only contained Fe2O3.The adsorption of total mercury by biochars is a combination of physical and chemical adsorption,and the adsorption of methylmercury is mainly by chemical adsorption.Among them,in the total mercury solution of 1000 μg/L,the adsorption rate of BC600-2 is 96.16%,and the adsorption capacity is 1.91 mg/g.In the methyl mercury solution of 10 μg/L,the adsorption rate of BC750-2 is 66.30%,and the adsorption capacity is 13.26 μg/g,but in the 0.1 μg/L methylmercury solution,the adsorption rate of BC600-2 can be as high as 99.65%.Through the method of impregnation-pyrolysis,the waste biomass of Eichhornia crassipes is prepared into magnetic activated biochar,which can not only increase its specific surface area,degree of porosity,enrich surface active sites which be beneficial to the adsorption of toxic mercury,but also rich in magnetic components which be convenient to recycle from the water phase or solid phase.It provides technical material support for subsequent treatment of mercury pollution in sediments,and also opens up the commercial utilization value of these invasive plants. |
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