| Heavy metal pollution is a grave environmental problem now because toxic metals, such as lead, zinc, copper and cadmium, are among the most common pollutants found in industrial effluents. Many industrial activities such as metal plating, fertilizer industry, mining operations, and textile introduce toxic metals to environment via their waste effluents. These heavy metals are high toxicity, nonbiodegradable and tend to accumulate in living organisms, causing harmful effects on human physiology and biological systems. Recently, phytoremediation by the use of aquatic plants is considered to be an ecologically acceptable technology for metal removal from the waste waters. Among various plant groups used for phytoremediation, Eichhornia crassipes has been paid much attention because of its fast growth rate and large uptake ability regarding to heavy metals. Eichhornia crassipes has numerous advantages to eliminate heavy metals in the waters, such as high efficiency, low-cost and sustainable which is of great significance to remove toxic metals and improve water quality. Many literatures show that the roots of Eichhornia crassipes are the key position to purify metals. It is hard for heavy metals to penetrate the cell wall of the roots which belongs to a semipermeable membrane so that metals are absorbed in the roots. Simultaneously, the contact areas between the roots of Eichhornia crassipes and water surface are large enough to form a dense filter layer which leads a lot of metals to adsorb on the roots.Long-root Eichhornia crassipes were breeded through spraying inducers onto Eichhornia crassipes by Institution of Agriculture and Ecology in Yunnan Province. In comparison with Eichhornia crassipes, the leaves of Long-root Eichhornia crassipes were only one tenth of the Eichhornia crassipes. But the roots of Long-root Eichhornia crassipes were about 80 cm or more than 1 m, which was benificial to remove heavy metals quickly. Removal of Pb, Zn, Cu and Cd by two kinds of Eichhornia crassipes was compared firstly and the roots and biochars derived from Long-root Eichhornia crassipes were further studied according to its superior performance on removing metals. The absorption and adsorption processes were investigated and discussed through experiments and modern analysis methods which provided a scientific basis for cultivation of Long-root Eichhornia crassipes in Dianchi Lake and the possibility of resource reuse for abandoned biomass from Long-root Eichhornia crassipes,A comparative study on the removal of toxic metals from aqueous solution by Long-root Eichhornia crassipes and Eichhornia crassipes was conducted. On the whole, Long-root Eichhornia crassipes had a higher relative growth rate and removal efficiency than Eichhornia crassipes. Meanwhile, The solution had a better physicochemical property after dealing with Long-root Eichhornia crassipes. In single-metal system, the removal percentages of Pb, Zn, Cu and Cd were all more than 80% within 4 days and followed the order of Pb>Cd>Cu>Zn. In multi-metal system, Long-root Eichhornia crassipes had a better removal percentage on Pb and was not affected by the other three metals; There were obviously effects on the elimination of Zn when Pb, Cu and Cd coexisted; The adsorption process of Cu2+ was affected by Pb and Cd and the adsorption process of Cd 2+ was affected by Pb, Zn and Cu. The roots of Long-root Eichhornia crassipes were the key part to remove metals through adsorption on the surface of roots and palnt absorption. The removal percentages through adsorption on the roots were 75%,43%,29%and 61%for Pb, Zn, Cu and Cd, respectively. And among the adsorption, the removal percentages through ion exchange were 17%、13%、18% and 20% for Pb, Zn, Cu and Cd, respectively. The absorption was also occurred and bioconcentration factor of Pb, Zn, Cu and Cd were 267,102,230 and 173 with Long-root Eichhornia crassipes, respectively.SEM images showed that the root pores of Long-root Eichhornia crassipes didn’t present a clear crystal form and its surface was irregular rectangle. Energy spectrum analysis showed that the metals were adsorbed on root powders after experiment. BET results showed that the specific surface area and average pore width were 1.1 m2/g and 11.14 nm. The observed hysteresis belong to type H3, which was indicative of narrow cracks and formed by the accumulation of flake particle. From FT-IR, we found that the absorption peaks of phosphorous compounds, carboxyl, and nitrogenous compounds displayed obvious changes before and after adsorption which illustrated that plant characteristics may play a role in binding with metals. In single-system, the removal percentage of metals was in the order of Pb>Cd>Zn>Cu when the quantity of powder was 20 g/L and pH was 5. Langmuir model and the second-order kinetics equation could well described the adsorption process of heavy metals in single-metal system. In multi-metal system, the adsorption performance of Pb, Zn and Cd in the multi-metal system decreased with the concentration increased. At last, competitive adsorption of every two metals on root powders proved that Cu and Pb had suppressed the adsorption performance of Cd and Zn.Biochars were produced from Long-root Eichhornia crassipes at four temperatures: 200°C,300°C,400°C and 500°C under O2 limited condition, referred to as LEC200, LEC300, LEC400 and LEC500, respectively. The content of carbon in biochars increased and the content of oxygen decreased with increasing temperature, but the nitrogen content of the four chars were quite similar. In addition, the sorption process of four metals by the biochars in different metal concentration were all excellently represented by the psudo-second-order model. LEC500 was the best one to bind heavy metals due to its highest alkalinity and largest specific surface area. At room temperature, the qmax of Pb, Zn, Cuand Cd were 39.09 mg/L,45.40 mg/L,48.20 mg/L and 44.04 mg/L, respectively. The sorption process of four metals in different temperatures could be described satisfactorily by the Langmuir isotherms according to R2. The positive values of AH0 and AS0, the negative values of AG° confirmed the endothermic and spontaneous nature of adsorption. All the biochars had a lower CEC values but a higher pH values. pH values decreased quickly after adsorbing metals illustrated that chemical precipitation in the process of adsorption was more important than ion exchange. Furthermore, it was also found that the alkyl, carboxyl, phosphate and cyano groups in the biochars can play a role in adsorbing metals according to FT-IR.In total, Long-root Eichhornia crassipes has a better performance than Eichhornia crassipes in removing Pb, Zn, Cu and Cd from aqueous solutions. The root powders and biochars derivied from Long-root Eichhornia crassipes for recycling have good performance as well in adsorption of Pb, Zn, Cu and Cd, which provides a new method and idea for removing metals in single and multi metal system. |
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