岩溶地下河反硝化作用的有限性——一个碳酸盐岩管道的实验研究

反硝化作用是公认的去除水体中硝酸盐的路径,但相比于多孔介质,岩溶地下河中反硝化效果具有不确定性.为更好地认识岩溶地下河中反硝化效果,本研究利用天然碳酸盐岩管道几何模型,以乙醇为可利用电子供体(碳源),示踪了控制流速条件下管道流中反硝化作用,并辅以多孔介质流实验进行对比.反应示踪、地球化学印迹和微生物检测结果表明:当碳源缺乏时,反硝化作用没有明显启动的迹象;一旦补充了碳源,溶解氧、硝酸盐浓度和质量都出现了明显衰减,并且有中间产物亚硝酸盐产生,水体碱度增加.然而,即使在碳源充足情况下,管道流中反硝化强度却明显比多孔介质流中强度低,两者硝酸盐生物去除率分别为39.4%和大于99%,生物降解速率分别为0.113和10.8 mg·L~(-1)·h~(-1).推测其原因,一是碳酸盐岩管道内固体表面积与水体积比值低,固体吸着条件不利于微生物生长与发育,降低了硝酸盐去除率;二是管道富含的溶解氧可能延迟了反硝化作用启动,溶解氧降至3.0 mg·L~(-1)左右时硝酸盐浓度才有明显衰减.相比之下,其它环境因素如p H值和温度没有出现明显变化.该研究意义在于:岩溶管道流反硝化去除硝酸盐的潜能是存在的,但即使可利用碳源充足仍具有明显的局限性,这可能意味着岩溶地下河一旦遭受硝酸盐污染,其作为饮用水源的安全风险更大.

Limitation of denitrification in karst subterranean river: A carbonate-conduit experimental study

Denitrification is generally recognized as a path to remove nitrate in water, but compared with porous media, the effectiveness of denitrification in karst subterranean river is uncertain. In order to better understand the question, the denitrification in a geometric carbonate-conduit model was traced under the controlled flow condition and was compared with the experiment in porous media. The results indicate that there was no obvious denitrification when carbon source was lacking. Once ethanol was added as an available carbon source, the concentrations and qualities of both dissolved oxygen (DO) and nitrate were significantly attenuated. Moreover, nitrite as an intermediate occurred and the water alkalinity increased. However, in the case of carbon sources, the intensity of denitrification in the carbonate-conduit flow was significantly lower than that in porous media, even though carbon source was fully recharged. The two removal rates of nitrate were 39.4% and >99%, and the biodegradation rates were 0.113 and 10.8 mg·L^-1·h^-1, respectively. It was deduced that the ratio of solid surface area to water volume within the conduit was lower, so that the microbial solid-attached condition was decreased, which exerted impact on the microbial growth and development as well as the removal rate of nitrate. Additionally, rich DO in the conduit flow might delay the onset of denitrification. Nitrate concentrations attenuated significantly when DO dropped to about 3.0 mg·L^-1. In contrast, other environmental factors including pH and temperature had no significant change. This study highlights that bio-removal of nitrate by denitrification is of potential but with limitation in karst subterranean river even if available carbon source is adequate, which may increase the safety risk of drinking water.