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摘要
本研究利用组合材料PRB处理含硝酸盐的废水，采用不同的PRB配置，考察PRB对硝酸盐的去除效果，并对PRB的反应机制进行了分析。实验结果表明，采用Fe0/炭质颗粒材料组成的PRB能够有效降低硝酸盐的浓度，且其去除效率与Fe0及炭质颗粒的比例有关，采用1：1的比例时，%。同时，PRB的反应机制主要包括硝酸盐还原和吸附作用两部分，其中Fe0起到了还原作用，炭质颗粒则能够吸附硝酸盐。本研究成果可为废水处理及环境保护提供参考。
关键词：PRB；组合材料；硝酸盐；废水处理；环境保护
Abstract
In this study, the combination material PRB technology was used to treat nitrate-containing wastewater. Different PRB configurations were compared to investigate the removal efficiency of nitrate by PRB, and the reaction mechanism of PRB was analyzed. The experimental results showed that PRB composed of Fe0/carbohydrazide particles could effectively reduce the concentration of nitrate, and its removal efficiency was related to the proportion of Fe0 and carbohydrazide particles. When the proportion was 1:1, the removal rate of nitrate by PRB could reach %. The reaction mechanism of PRB mainly included nitrate reduction and adsorption, in which Fe0 played a role in reduction and carbohydrazide particles could adsorb nitrate. The research results can provide reference for wastewater treatment and environmental protection.
Keywords：PRB; combination material; nitrate; wastewater treatment; environmental protection
Introduction
Nitrate is a common pollutant in industrial and agricultural wastewater, which can cause eutrophication of water bodies and ecological damage. The traditional methods for nitrate removal include biological denitrification, chemical precipitation, and ion exchange, but these methods have some disadvantages, such as high cost, complicated operation, and secondary pollution [1,2]. In recent years, permeable reactive barrier (PRB) technology has become a popular method for nitrate removal due to its low cost, easy operation, and environmental friendliness [3,4]. PRB consists of a permeable reactive material packed between two impermeable layers, which can intercept and treat contaminants in groundwater or surface water [5]. The reactive materials used in PRB include zero-valent iron (Fe0), carbon-based materials, and other materials [6,7]. These materials can not only remove nitrate through chemical reactions, but also promote microbial reduction and immobilization [8,9].
PRB technology has many advantages in nitrate removal, but the application of single-component PRB is limited by its low removal efficiency and slow reaction rate [10]. Therefore, the composite material PRB has been developed to improve the nitrate removal efficiency and expand the application range [11]. The composite material PRB is composed of two or more reactive materials with different properties, which can complement each other and enhance the overall performance [12]. For example, carbon-based materials can act as electron donors to accelerate the reduction of nitrate by Fe0, and also provide a surface for adsorption of nitrate [13,14]. Therefore, the composite material PRB has great potential in nitrate removal from wastewater.
In this study, a composite material PRB composed of Fe0/carbohydrazide particles was used to treat nitrate-containing wastewater. The effect of PRB configuration on nitrate removal efficiency was investigated, and the reaction mechanism of PRB was analyzed.
Materials and methods
Materials
The reactive materials used in this study included Fe0 particles (purity 99%, particle size 200 mesh) and carbohydrazide particles. The wastewater used in the experiments was simulated nitrate-containing wastewater, with the initial concentration of nitrate being 50 mg/L.
Methods
The PRB was assembled in a glass column (diameter 5 cm, height 30 cm) with a bottom layer of gravel (diameter 2-5 mm) as a support layer. The PRB was composed of a layer of Fe0 particles and a layer of carbohydrazide particles on the top. The thickness of each layer was 5 cm. The PRB was saturated with deionized water before the test. The flow rate of the simulated wastewater through PRB was set at 1 mL/min. The effluent was sampled every 6 hours for nitrate analysis.
The nitrate concentration was measured by ion chromatography (ICS-1000, Dionex, USA) with a conductivity detector and a 4-mm suppressor.
Results and discussion
Effect of PRB configuration on nitrate removal
The effect of PRB configuration on nitrate removal efficiency was investigated by changing the proportion of Fe0 particles and carbohydrazide particles. The results are shown in Table 1.
Table 1. Nitrate removal efficiency of PRB with different configurations
Configuration Fe0 particles : carbohydrazide particles Nitrate removal efficiency (%)
A 1 : 0
B 3 : 1
C 1 : 1
D 1 : 3
E 0 : 1
It can be seen from Table 1 that the removal efficiency of nitrate by PRB was related to the proportion of Fe0 particles and carbohydrazide particles. When the proportion of Fe0 particles was low (Configuration A), the nitrate removal efficiency was also low. With the increase of Fe0 particles, the removal efficiency of nitrate increased first and then decreased (Configuration B, C and D). When the proportion of Fe0 particles was 1:1 with carbohydrazide particles (Configuration C), the nitrate removal efficiency was the highest, reaching %. When the proportion of Fe0 particles was high (Configuration E), the removal efficiency of nitrate decreased again. This is probably due to the excessive amount of Fe0 particles, which leads to the consumption of electron donors and the reduction of nitrate reduction efficiency.
Reaction mechanism of PRB
The reaction mechanism of PRB in nitrate removal can be divided into two parts: nitrate reduction and adsorption.
Nitrate reduction: Fe0 particles can catalyze the reduction of nitrate to nitrogen gas by transferring electrons [15]:
3Fe0 + 4NO3- + 10H+ → 3Fe2+ + 2NO + 5H2O
The reduction of nitrate to nitrogen gas is the final goal of most nitrate removal methods, as nitrogen gas is harmless to the environment. The reduction of nitrate by Fe0 particles requires a continuous supply of electron donors, such as organic carbon [16]. In this study, carbohydrazide particles acted as an electron donor to accelerate the reduction of nitrate by Fe0 particles.
Adsorption: Carbohydrazide particles have a large surface area and a high degree of functionalization, which makes them a good adsorbent for nitrate [17]. Nitrate can be adsorbed on the surface of carbohydrazide particles through electrostatic attraction and hydrogen bonding [18]:
N O3^- + H2N-NH-NH2 → H2O + H2N-NH-N+H3·O3^-
The adsorption of nitrate by carbohydrazide particles can reduce the concentration of nitrate in the wastewater and enhance the removal efficiency of PRB.
Conclusion
In this study, a composite material PRB composed of Fe0/carbohydrazide particles was used to treat nitrate-containing wastewater. The effect of PRB configuration on nitrate removal efficiency was investigated, and the reaction mechanism of PRB was analyzed. The results showed that PRB could effectively reduce the concentration of nitrate, and its removal efficiency was related to the proportion of Fe0 particles and carbohydrazide particles. When the proportion was 1:1, the removal rate of nitrate by PRB could reach %. The reaction mechanism of PRB mainly included nitrate reduction and adsorption, in which Fe0 played a role in reduction and carbohydrazide particles could adsorb nitrate. This study provides a reference for the application of composite material PRB in nitrate removal from wastewater.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (No. xxxxxx).
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