Design considerations for groundwater remediation using reduced metals

Abstract

Use of reduced metals has attracted much attention since it possesses a great potential for eliminating reducible contaminants in groundwater such as heavy metals and chlorinated compounds. However, products of metal-mediated reactions for many chlorinated hydrocarbons have not clearly been identified. In addition, consumption of the metals, generation and release of metal ions, formation of insoluble metal oxides and hydroxides on the clean metal surface, and rise of pH inevitably accompany the reactions. Due to these properties of metal-mediated reactions, the reaction rate could decrease as the reaction proceeds, and effluent quality could decay. It was shown in this study using chlorine mass balance and GC analysis that chloroform is formed from carbon tetrachloride by reduced iron. It is also well-known that nitrate is reduced mostly to ammonia by metals, which indicates that the metal process is inappropriate for denitrification of nitrate-contaminated aquifers. These results indicate that groundwater remediation using metal process requires careful consideration for the safety of reaction products. It was also shown that mixing rate strongly affects reaction rate since metal-mediated reaction occurs on the surface of metals. In addition, reaction rate was decreased due to metal hydroxide deposition on the surface of metal granules that was seen by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. Generation of iron ions (consumption of reduced iron) released from reduction of zero-valent iron was also shown by using an ion chromatograph (IC). In this study, some methods were suggested to solve the above-mentioned problems. Acid washing appeared effective for removing corrosion products on the surface of metal granules, by which a reduction rate could be maintained high for an extended time of reaction. Use of iron sulfide decreased an extent of pH rise during metal-mediated reaction; thereby precipitation of insoluble metal (hydr)oxides is expectedly decreased. It was also shown that inexpensive iron scrap instead of fine metal powders can be used for metal processes.