Influencing factors and performance of enhanced denitrification layer in the vadose zone

doi:10.13745/j.esf.sf.2021.2.6

Abstract

Abstract:

The vadose zone is a natural barrier for preventing nitrate contamination of groundwater and its denitrification effect is limited by carbon contents. In order to control nitrate pollution in groundwater, the Ca(OH)₂-treated corn cobs were used as a carbon source to construct a reactive layer to speed up denitrification. The effects of interactions between nitrate concentration, humidity and temperature on denitrification performance were studied by response surface method. Denitrification performance was then comprehensively evaluated in terms of nitrate removal efficiency, nitrite accumulation concentration, pH value and leaching of dissolved organic carbon (DOC). The results show that the temperature and humidity as well as the interactions between them had significant effects on nitrate removal efficiency, with temperature being the most critical factor in this process. After 74 days, the nitrate removal efficiency reached 50% and the nitrite level was mostly lower than 3 mg(N)/L; the pH of the reactive layer maintained around 7.0; and the DOC leaching flux fluctuated between 0.1 and 0.2 mg(C)/(cm²·d). A high-throughput sequencing analysis shows an decrease of microbial richness in the denitrification layer and an increase of relative abundances of microorganisms related to denitrification and carbon source decomposition. The aborigine bacteria in the vadose zone are abundant, which is beneficial for denitrification under the effect of carbon source.

Key words: vadose zone, carbon source, denitrification, high throughput sequencing, response surface

CLC Number:

P641.2

SUN Zhaoyue, ZHENG Xilai, ZHENG Tianyuan, LUAN Yongxia, XIN Jia. Influencing factors and performance of enhanced denitrification layer in the vadose zone[J]. Earth Science Frontiers, 2021, 28(5): 136-145.

Figures/Tables 9

Table 1 Box-Behnken design experiments and response values

试验设计	影响因素				响应值
试验设计	温度A/℃	硝态氮浓度 B/(mg·L^-1)	含水量C (WHC)/%		硝态氮去除率Y/%
1	10	100	80	24.00
2	30	100	80	61.42
3	10	300	80	27.1
4	30	300	80	43.01
5	10	200	60	28.02
6	30	200	60	35.00
7	10	200	100	30.43
8	30	200	100	100.00
9	20	100	60	30.74
10	20	300	60	32.20
11	20	100	100	40.72
12	20	300	100	33.30
13	20	200	80	33.45
14	20	200	80	33.44
15	20	200	80	33.81
16	20	200	80	33.80
17	20	200	80	36.01

Fig.1 Schematic diagrams of experimental setup (a) and the enhanced denitrification layer (b)

Fig.2 Response surface plots showing the effects of interactions between nitrate-nitrogen concentration and temperature (a), moisture and temperature (b) or moisture and nitrate-nitrogen concentration (c) on the nitrate-nitrogen removal efficiency

Table 2 Variance analysis of nitrate nitrogen removal efficiency

项目	平方和	自由度	平均方差	F值	p值
模型	4 766.76	9	529.64	13.83	0.001 1
温度A	1 796.70	1	1 796.70	46.93	0.000 2
硝态氮浓度B	46.46	1	46.46	1.21	0.307 0
含水量C	552.95	1	552.95	14.44	0.006 7
D_AB	138.30	1	138.30	3.61	0.099 1
E_AC	1 390.17	1	1 390.17	36.31	0.000 5
F_BC	19.71	1	19.71	0.51	0.496 2
A²	482.04	1	482.04	12.59	0.009 4
B²	173.42	1	173.42	4.53	0.070 8
C²	181.18	1	181.18	4.73	0.066 1

Fig.3 Changes of nitrate-nitrogen concentration (a), nitrite-nitrogen concentration (b), pH value (c) and DOC leaching flux (d) in the effluent of denitrification layer

Table 3 Analytical results of microbial community diversity in the denitrification layer and soil control group

样品名称	OTUs	Shannon	Simpson	Chao1	ACE	Good’s coverage
脱氮层	953	6.941	0.984	946.485	992.522	0.992
土壤对照组	1 413	7.101	0.938	1 411.484	1 414.006	0.992

Fig.4 Relative abundances of predominant microbial phyla in the denitrification layer (a) and soil control group (b)

Fig.5 Relative abundances of predominant bacterial classes in the denitrification layer (a) and soil control group (b)

Fig.6 Relative abundances of predominant functional bacterial genera in the denitrification layer (a) and soil control group (b)

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