Influence of electrolyte and voltage on the direct current enhanced transport of iron nanoparticles in clay

doi:10.1016/j.chemosphere.2013.10.065

Chemosphere

Volume 99, March 2014, Pages 171-179

https://doi.org/10.1016/j.chemosphere.2013.10.065 Get rights and content

Highlights

•
Direct current can enhance iron nanoparticles transport in clay by 25%.
•
Oxidizing conditions and higher ionic strength limit nZVI enhanced transport.
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Ionic strength was significant, promoting nanoparticles aggregation and oxidation.

Abstract

Zero valent iron nanoparticles (nZVI) transport for soil and groundwater remediation is slowed down or halted by aggregation or fast depletion in the soil pores. Direct electric current can enhance the transport of nZVI in low permeability soils. However operational factors, including pH, oxidation–reduction potential (ORP), voltage and ionic strength of the electrolyte can play an important role in the treatment effectiveness. Experiments were conducted to enhance polymer coated nZVI mobility in a model low permeability soil medium (kaolin clay) using low direct current. Different electrolytes of varying ionic strengths and initial pH and high nZVI concentrations were applied. Results showed that the nZVI transport is enhanced by direct current, even considering concentrations typical of field application that favor nanoparticle aggregation. However, the factors considered (pH, ORP, voltage and electrolyte) failed to explain the iron concentration variation. The electrolyte and its ionic strength proved to be significant for pH and ORP measured during the experiments, and therefore will affect aggregation and fast oxidation of the particles.

Introduction

Zero valent iron nanoparticles (nZVI) are considered an emergent solution for in situ soil and groundwater remediation due to their high specific surface area and reactivity and because they target a vast number of contaminants, from organochlorines to heavy metals (Masciangioli and Zhang, 2003, Zhang, 2003, Zhang and Elliott, 2006). The growing use of nZVI in pilot and full-scale applications in the last decade is notable (USEPA, 2011, Mueller et al., 2012, Rejeski et al., 2012). However, one of the major limitations is the effective long distance transport without aggregation and loss of their reactivity – the mobility of nZVI in the subsurface is normally less than a few meters (Bennett et al., 2010, He et al., 2010, Comba et al., 2011, Su et al., 2013). Coated nanoparticles are more mobile than bare nZVI (He et al., 2007, Phenrat et al., 2007, Sun et al., 2007, Kanel et al., 2008, Tiraferri et al., 2008, Lin et al., 2009, Phenrat et al., 2009, Tiraferri and Sethi, 2009, Phenrat et al., 2010, Raychoudhury et al., 2010, Jiemvarangkul et al., 2011, Phenrat et al., 2011), but aggregation remains and can be determined by the particle size distribution and Fe⁰ content of nZVI, as well as by soil water ionic strength and composition (Saleh et al., 2008, Lin et al., 2010).

Electrokinetic (EK) remediation is a well-known technology with demonstrated results, especially in low permeability fine-grain soils. Direct current can enhance the transport of iron nanoparticles in sands (Jones et al., 2010, Chowdhury et al., 2012) and clay (Pamukcu et al., 2008) and improve the remediation effectiveness for different contaminants (Yang et al., 2008, Reddy et al., 2011, Gomes et al., 2012a, Gomes et al., 2012b, Yuan et al., 2012, Fan et al., 2013). The primary mechanisms for the nZVI enhanced transport are: electrophoresis, towards the anode, and electroosmotic advection, towards the cathode. In sands, with lower surface charge, electrophoresis dominates, while in clays, electroosmosis can be the most important transport mechanism, counteracting electrophoresis. The existing studies tested low nZVI concentrations and overlooked parameters such as ionic strength, pH, oxidation–reduction potential (ORP) and electric field strength that can influence the nZVI transport, given that the external supply of electric energy can enhance favorable oxidation–reduction reactions in clay-electrolyte systems (Pamukcu, 2009).

The main objective of the current study was to assess if low direct current can enhance the transport of high nZVI concentrations, typical of field applications, in clay rich soils varying the electrolyte ionic strength and voltage. We used kaolin clay to represent a low permeability medium, and an experimental setup that allowed us to study the variation in the oxidation–reduction potential and pH values in the kaolin during short-term experiments, and estimate the temporal and spatial distribution of the iron oxidation states and hence the reactivity of the nanoparticles.

Section snippets

Chemicals

Before synthesis of the iron nanoparticles, deionized (DI) water was purged with ultra-purified grade nitrogen gas (N₂) for 1 h so that dissolved oxygen would fall to a level below 20%. Iron nanoparticles were prepared reducing FeSO₄·7H₂O (MP Biomedicals), dissolved in a polyacrylic acid, sodium salt – (PAA) Mw ∼2100 (Polysciences, Inc.) solution, by sodium borohydride (Hydrifin™), using the procedure described by Kanel et al. (2008). The PAA–nZVI suspensions were freshly prepared before each

Enhanced transport of iron nanoparticles

In general, higher iron concentrations were measured when direct current was applied (Fig. 2), indicating an enhancement of nZVI transport over diffusion in kaolin clay. An average concentration increase of 25% was observed when using high concentrations of nZVI, typical of field applications. The Fe concentrations obtained in the enhanced transport tests are statistically different from the diffusion tests at a 0.05 level of significance [one-way ANOVA, F(1,38) = 5.04, p = 0.03]. Use of statistic

Conclusions

The use of direct current enhanced the nZVI transport in the kaolin, using high concentrations, typical of field applications. However, the iron concentration variability could not be explained by pH, ORP, voltage and electrolyte. In the variation of pH and ORP during the experiments, the electrolyte and its ionic strength proved to be significant, and thus will have affected aggregation and fast oxidation of the particles. Clear distinctions were observed in ORP distribution between

Acknowledgments

This work has been funded by FP7-PEOPLE-IRSES-2010-269289-ELECTROACROSS, by Portuguese National funds through “FCT – Fundação para a Ciência e a Tecnologia” under project PTDC/AGR-AAM/101643/2008 NanoDC and by the research grant SFRH/BD/76070/2011. The Department of Civil and Environmental Engineering at Lehigh University is acknowledged where all the equipment development, testing and analysis were funded.

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Influence of electrolyte and voltage on the direct current enhanced transport of iron nanoparticles in clay

Highlights

Abstract

Introduction

Section snippets

Chemicals

Enhanced transport of iron nanoparticles

Conclusions

Acknowledgments

J. Contam. Hydrol.

J. Hazard. Mater. B

Adv. Water Resour.

Colloid Surface A

Chemosphere

Water Res.

Chem. Eng. J.

J. Colloid Interface Sci.

Colloid Surface A

Sci. Total Environ.

J. Contam. Hydrol.

Separ. Purif. Technol.

Colloid Surface A

J. Colloid Interface Sci.

Sep. Purif. Technol.

Separ. Purif. Technol.

Geoderma

Electrochemical Methods: Fundamentals and Applications

A comparison between field applications of nano-, micro-, and millimetric zero-valent iron for the remediation of contaminated aquifers

Water Air Soil Poll.

Field assessment of nanoscale bimetallic particles for groundwater treatment

Environ. Sci. Technol.

Electrokinetic enhanced transport of zero valent iron nanoparticles for chromium (VI) reduction in soils

Chem. Eng. Trans.

Stabilization of Fe–Pd nanoparticles with sodium carboxymethyl cellulose for enhanced transport and dechlorination of trichloroethylene in soil and groundwater

Ind. Eng. Chem. Res.