Adsorption and dechlorination of 2,4-dichlorophenol (2,4-DCP) on a multi-functional organo-smectite templated zero-valent iron composite

Abstract

Controlling the reactivity of nano-scale zero-valent iron (nZVI) remains a challenge for its practical application. In the present study, smectite-templated nZVI is hydrophobized by adding N,N,N-trimethyl-1-dodecanaminium salt (DTA⁺) to yield organo-smectite-ZVI. The obtained material was characterized by XRD, TEM and FTIR. Its reactivity was evaluated for the aqueous removal of 2,4-DCP. Results show that (i) nanosized ZVI clusters of <5 nm are intercalated into the clay interlayers; (ii) hydrophobization of smectite surfaces occurs after binding of DTA⁺ to the clay minerals; (iii) aqueous 2,4-DCP could be rapidly accumulated in the vicinity of the solid phase; (iv) accumulated 2,4-DCP is then gradually dechlorinated. This demonstrates that hydrophobic conditions in clay interlayer facilitate the 2,4-DCP adsorption. In a 2,4-DCP successive addition systems, dechlorination can be maintained even after five cycles for organo-smectite-ZVI, but just two cycles for smectite-ZVI. This indicates that the hydrophobization of smectite-ZVI could significantly sustain its reactivity and inhibit the rapid consumption of ZVI in the Fe⁰/H₂O system. This statement is supported by XPS analysis. Furthermore, organo-smectite-ZVI provides strong adsorptive affinity to 2,4-DCP and its reaction products. This is beneficial for the long-term stability of removed contaminants.

Introduction

Nanosized zero-valent iron (nZVI) has been successfully synthesized and utilized as a removing agent to treat contaminants, such as nitroaromatic compounds, halogenated hydrocarbons, and trace metals in subsurface water for over a decade [1], [2], [3], [4], [5], [6], [7]. An attractive property for ZVI particle size getting into nano region is that the ratio of surface to volume is substantially increased, thus facilitating the removal of recalcitrant contaminants and offering the possibility of subsurface employment via injection [2], [3], [5], [7]. However, a few drawbacks remain in its synthesis and practical application. Firstly, the high reactivity of nZVI induces its quick consumption during synthesis, storage and application, resulting in the depletion of reducing agents (such as Fe(II), H and H₂) in short time [6], [7], [8]. This chemical instability has been considered as a main reason for the observed low efficiency of nZVI systems compared with that of μm- and mm-ZVI [6], [7]. Secondly, expansive corrosion is a universal property of metals (such as nZVI). Under oxic conditions, this corrosion could be accelerated and resultant products are larger in volume (V_Fe3O4/V_Fe = 2.1, V_oxide/V_Fe up to 6.4) [9], which is detrimental to successful mass transfer of removal agent to contaminants from bulk media [3], [6]. Thirdly, reaction products retaining on the ZVI surface will decrease the number of available surface active sites, resulting in lower activity. Thus, the removal of products and prevention of surface deactivation are necessary for practical application of ZVI [10], [11]. In addition, the organic contaminants are generally not completely mineralized by nZVI, thus their products are released into bulk solution and some of them will induce secondary pollution. For in situ field applications, it is thereby desirable to remove contaminants and their subsequent products completely.

Surface-modification has been applied for stabilizing nZVI particles and efficiently sustaining their reactivity. Typical agents such as carboxymethyl cellulose, polyvinylpyrrolidone, starch, polyelectrolyte membranes, and guar gum have been employed in an effort to enhance the suspension stability and therefore particle mobility of nZVI during preparation and utilization [12], [13], [14], [15], [16]. These stabilizations are achieved by enhancing the steric or electrostatic repulsions between the particles to inhibit nZVI aggregation and hence to increase the stability. In addition, introduction of low reactive material such as sand or natural minerals into ZVI is an effective tool to prevent compaction and delay clogging, which increases the removal efficiency of contaminants by nZVI [10], [11]. In this context, smectite has been employed as a supporting matrix to synthesize stabilized ZVI in the interlayer [17], [18]. Smectite clay layers generally consist of a center octahedral AlO sheet sandwiched between two tetrahedral SiO sheets. The planar aluminosilicate layers typically exist in stacked assemblages. Negative charges are embedded in the individual layers (due to isomorphic substitution) in a fixed but isolated distribution, and commonly neutralized by exchangeable inorganic cations that reside at or near the negatively charged sites. Smectite clay usually features with an interlayer distance (between two clay sheets) ranging from ∼0.1 to 0.8 nm. This structural arrangement creates an ideal template to host ZVI clusters formed by reduction of exchangeable Fe(III) [17], [18]. In this approach, smectite clay functions as a layered template that constrains the size of the ZVI formed at the nanoscale, and directs a fixed distribution of isolated regions of the ZVI clusters, which enhances the stability of the ZVI due to the readily dispersion property of smectite clay [17].

Based on our previous work [17], [18], hydrophobic composite system benefits the adsorption of target organic compounds and products, and also potentially acts as a retardant against rapid consumption of nZVI by water and dissolved O₂, thus to increase its efficiency and to inhibit volumetric expansion [7]. Organic modification of smectite interlayer has been proven to be an effective strategy to enhance the interlayer hydrophobization and adsorption capacity by organic cations for ion exchange [19], [20]. From this prospect, organic modified smectite-templated nZVI composites are developed for effective removal of persistent organic pollutants (such as 2,4-DCP, which is persistent to nature degradation and abundance in environmental media) in the present work. Furthermore, the combined adsorption–dechlorination property and involved mechanism of as-synthesized composite are studied in detail, including the efficacy of nZVI loaded into organic modified interlayer environment and the adsorption properties of organo-smectite-ZVI for products.