He says the movement of rivers usually removes material like nitrates.
2
所以他想弄明白，为什么自然过滤过程未能去除密西西比河的硝酸盐。
So he wondered why that natural filtration process does not remove nitrates from the Mississippi River.
3
该标准声明：柠檬酸-硝酸钠处理在去除游离铁合其他金属污染和轻的表面污染物时，危险性最低。
The standard states that the citric acid-sodium nitrate treatment is the least hazardous for removal of free iron and other metallic contamination and light surface contamination.
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