Stabilized chitosan/Fe⁰-nanoparticle beads to remove heavy metals from polluted sediments

In the last few decades, sediment contamination by heavy metals, such as Cr(VI), Cd(II), Pb(II), etc., has been a major concern to environmental safety and public health (Arienzo et al. 2013). The interaction of heavy metals with sediment is complex (Hsieh et al. 2007; Orecchio & Giuseppe 2013; Bo et al. 2015), and sediment sorption can inhibit or limit the removal of heavy metals by sorption agents. Moreover, heavy metals cannot be degraded but are easily accumulated in human tissue (Akcil et al. 2015). Biological magnification through the food chain is a major concern for human health and environmental safety (Singh et al. 2000). Thus, remediation technologies are required to understand this matter more fully.

Recently, nanoscale zero-valent iron (NZVI) has been introduced into reducing various contaminants from soil and wastewater (Li & Zhang 2006; Zhang et al. 2011). However, the delivery and mobility of NZVI in soil limited the application of NZVI (He et al. 2007). NZVI in an aggregate state prevented its movement through sand and soil (Kanel & Choi 2007). To improve the delivery and mobility of NZVI in soil, various surfactants, such as starches (He & Zhao 2005) and carboxymethyl cellulose (He et al. 2009), have been explored. Compared to non-stabilized NZVI, the stabilized NZVI displayed much improved soil mobility (Zhang et al. 2011).

Most reported studies have focused on the modification of NZVI to improve its delivery and mobility in soil. Due to their small size, NZVI was hardly separable from sediments after the remediation. However, the knowledge of the separation of NZVI from sediment after remediation has been lacking.

In recent years, environmentally friendly materials have been used in wastewater treatment (Barreca et al. 2014a, b). Chitosan (CS) was also used to modify and support NZVI (Geng et al. 2009). The CS beads, especially, were porous and effective in removing heavy metal, because of their special structural properties and high adsorption capability (Lasko & Hurst 1999; Li et al. 2005). However, the mechanical strength of CS beads needs to be improved (Candy & Sharma 1993). The mechanical strength of CS beads was obviously enhanced by glutaraldehyde (GLA) (Dambies et al. 2001; Wan-Ngah et al. 2002). However, the information on chemical modification of CS-NZVI beads by GLA is limited.

The present study has therefore been established not only to enhance the mechanical strength of CS-NZVI beads but also to improve the separation of NZVI from sediment after remediation. Multiple heavy metals solutions were used as test contaminants to examine the effectiveness of GLA-CS-NZVI beads to remediate polluted sediment.

The sediment collected from River Haihe (Tianjin, China) was air-dried at room temperature and then gently ground to pass through mesh screens. The properties of the sediment are shown in Table 1. It can be seen that the main part of the sediment was sand and the concentrations of heavy metals were much lower than those of the prepared sediment. A total of 20 g of the sediment was mixed with 100 mL of deionized water. A known mass of K₂Cr₂O₇, CdCl₂ and PbCl₂ was added into the prepared sediment-water mixture. The concentrations of Cr(VI), Cd(II) and Pb(II) in the sediment were 40 mg/kg, 46 mg/kg and 54 mg/kg, respectively.

CS-NZVI beads were prepared according to the procedures described in detail elsewhere (Li & Bai 2005). Recently prepared CS-NZVI beads were suspended in 2.0 g/L GLA solution for 24 h (Wan-Ngah et al. 2002). The GLA-CS-NZVI beads were then intensively washed with deionized water and stored in deoxygenated deionized water for further use.

GLA-CS-NZVI beads were added into the prepared sediment-water mixture at room temperature with mechanical agitation for 48 h. The sediment-water mixture was then withdrawn using a 10 mL dispensable syringe and filtered through a 30 μm filter. In this process, the GLA-CS-NZVI beads were purposefully excluded from the mixture. The remainder on the filter was frozen for the following freeze drying and microwave digesting treatment in accordance with a previously reported procedure (Hoffmann & Patzold 2002; Sandroni et al. 2003). All experiments were performed in duplicate.

The morphological analysis of GLA-CS-NZVI beads was performed using a scanning electron microscope (SEM) (SEM, FEI Nova NanoSEM 230). The morphological analysis of NZVI was performed using a transmission electron microscope (TEM, FEI Tecnai G2 F20). The concentrations of heavy metals were measured using inductively coupled plasma-mass spectrometry (ICP-MS, Elan-9000, PE). The concentration of Cr(VI) in the solution was determined using a UV/visible spectrophotometer and by the diphenylcarbazine method (Ponder et al. 2000). Fourier transform infrared (FTIR) spectra for the CS-NZVI beads before and after being exposed to GLA were obtained using a Nexus FTIR spectroscopy.

The stability of GLA-CS-NZVI beads was tested using mechanical properties and solubility in the solution. The mechanical strength of CS-NZVI and GLA-CS-NZVI beads was determined using a similar procedure described by Guo et al. (2004). The results are shown in Table 2.

It can be seen from Table 2 that crumpling ratios of GLA-CS-NZVI beads were markedly reduced after CS-NZVI beads were cross-linked with GLA, indicating that the mechanical strength of GLA-CS-NZVI beads was obviously enhanced. The blending of GLA in CS improved the mechanical strength of the hydrogel beads (Li & Bai 2002). A Schiff's reaction occurred between GLA and CS beads (Dambies et al. 2001), which could increase the stability of CS beads (Hsien & Rorrer 1997). Crumpling ratios of GLA-CS-NZVI beads were smaller than those of ECH-CS-NZVI beads (Liu et al. 2012), meaning that the mechanical strength of GLA-CS-NZVI beads was greater than that of ECH-CS-NZVI beads.

It was shown that the cross-linked beads (GLA-CS-NZVI beads) were found to be insoluble in alkaline and neutral solutions especially, as well as in acidic solution (Table 3). Due to a primary amine group on CS, CS beads could be dissolved in acid solutions (Wan-Ngah et al. 2002). A Schiff's reaction occurred between aldehyde groups on GLA and amine groups on the CS beads, which could enhance the stability in acidic media (Dambies et al. 2001). A chemical cross-linking reaction between GLA and CS was found to enhance the stability of CS beads in acid solutions (Li & Bai 2005). As a result, GLA-CA-NZVI beads could be used in acidic, alkaline and neutral solutions, which would extend its applicable field for in situ remediation of environmental pollution.

The SE of GLA-CS-NZVI beads from the sediment-water mixture after remediation was studied and the results are shown in Table 4. The SE of CS-NZVI beads and GLA-CS-NZVI beads from the sediment-water mixture was enhanced to 95.8 and 96.5%, respectively (Table 4). However, NZVI was nonseparable from the sediment-water mixture (Table 4). This meant that NZVI supported on GLA-CS beads could enhance its SE from the sediment. The sterically hindered effect was the main driving force that resulted in the separation of GLA-CS-NZVI beads from the sediment (Kurahashi et al. 2006). Sterically hindered substrates enjoyed a broad scope and wide functional group tolerance (Yin et al. 2002). Steric hindrance was shown to control CO₂-amine reactions (Sartor & Savage 1983). The efficiency of the carbon dioxide cycling process could be improved by a series of amino acid salts with sterically hindered amine groups (Hook 1997). Faster reaction rates have now been achieved with sterically hindered chelating alkyl phosphine ligands (Hamann & Hartwig 1998). Furthermore, GLA-CS-NZVI beads could remain suspended and floated in or on the sediment-water mixture due to the effect of the buoyancy force (Rani et al. 2010).

Very often, variation in the salinity might lead to the release of heavy metals back to the aqueous phase (Singh et al. 2000; Jain 2004), which would influence the concentration distribution of heavy metals in sediment and wastewater. As mentioned above, the effect of salinity on the removal efficiency was introduced and the results are shown in Figure 3(b). The removal rates of all heavy metals from sediment were higher than 48.5% and increased with an increase of salinity (Figure 3(b)).

The effect of the particle size of the sediment on the removal efficiency of heavy metals was studied and the results are shown in Figure 3(c). The removal rates of heavy metals decreased with an increasing proportion of the smaller particles in the sediment (Figure 3(c)). NZVI was an adsorbent with higher sorption ability than the other material (Bolan et al. 2014), however, heavy metals could be easily adsorbed by the sediment particles. As a result, the competitive adsorption of heavy metals between the sediment particles and GLA-CS-NZVI beads became stronger as the sediment particles were smaller. Therefore, the decreased removal rates were observed as the proportion of smaller particles in sediment increased.

The results clearly indicate that GLA-CS-NZVI beads with an improved mechanical strength and stability in the solution were efficient to remove heavy metals from sediment. GLA-CS-NZVI beads may possibly offer a way to effectively use NZVI in many surface water or groundwater remediation situations.

GLA-CS-NZVI beads were successfully prepared and resulted in being effective in removing heavy metals from the sediment. After cross-linking with GLA, the mechanical strength, stability and SE of CS-NZVI beads were clearly improved. Different experimental conditions played a rather important role in the removal efficiency of heavy metals from the sediment. According to the results, the removal rates of heavy metals from sediment-wastewater mixture could be increased by changing the environmental conditions for the in situ remediation of contaminated sediment or wastewater.

This work was supported by the Innovation Team Training Plan of the Tianjin Education Committee (TD12-5037), National Natural Science Foundation of China (21307090) and Tianjin Municipal Natural Science Foundation of China (14JCZDJC41000).