Characteristics of Aggregate Size Distribution of Nanoscale Zero-Valent Iron in Aqueous Suspensions and Its Effect on Transport Process in Porous Media

Ruiqi Duan,Yanhui Dong,Qian Zhang

doi:10.3390/w10060670

Abstract

Bare nanoscale zero-valent iron (NZVI) particles in aqueous suspensions aggregate into micron to submicron sizes. The transport process of enlarged aggregates or multi-sized aggregates is different from that of nanoparticles. In this work, we performed aggregate size distribution analysis of NZVI suspension using a laser grain size analyzer and conducted a series of continuous injection column experiments with different injected NZVI concentrations. The results show that aggregates in NZVI suspensions range from submicron to submillimeter size and are mainly distributed around 5–9 μm and 50–100 μm. Quantitative calculation of iron transport and retention showed that the retained iron linearly correlates with injected concentration. The cross-section images revealed that clogging weakened from inlet to outlet. Furthermore, larger aggregates (>40 μm) appeared more often in the rising-declining stages of breakthrough curves, whereas small aggregates (<30 μm) dominated the steady stage. Indeed, relatively preferential flow facilitated the transport and discharge of both large and small iron aggregates. Straining of glass beads especially for the large iron aggregates resulted in a decline in breakthrough. Moreover, the blocking of attached and plugged iron prevented later retention of iron, resulting in a certain concentration of iron in the effluents. Our study provides greater insight into the transport of NZVI.

Highlights

Nanoscale zero-valent iron (NZVI), or iron nanoparticles, can remain as stable suspensions for long periods of time and form an in situ treatment zone when injected into groundwater
The unique size and characteristics of NZVI particles cause many difficulties in the groundwater remediation application of NZVI: (1) poor stability and transport in groundwater; and (2) growing particle size resulting from aggregation, and co-precipitation with products such as metal hydroxide, which leads to lowering of permeability due to clogging of pores
The the commercial powder had had an average size of nm, aggregation occurred upon dispersion in water and the aggregate size an average size of 50 nm, aggregation occurred upon dispersion in water and the aggregate significantly increased to the micron micron level

Summary

Introduction

Nanoscale zero-valent iron (NZVI), or iron nanoparticles, can remain as stable suspensions for long periods of time and form an in situ treatment zone when injected into groundwater. They have become the most attractive tool for in situ contaminated groundwater remediation owing to their large specific surface area and high reactivity with a broad range of targets such as organic compounds [1], metal ions [1,2], metalloids, and nitrates [3,4]. Previous studies mainly concentrated on the modification of NZVI with various kinds of materials, such as surfactants, to enhance its transport and stability, and involved a series of experimental and

Methods

Results

Conclusion