Chitosan nanoparticle (CS NP)-modified MnO2 nanoflakes were presented as a novel adsorbent for fast adsorption of Pb(II) from aqueous solution. Loading dense CS NPs onto mono-dispersive flower-like MnO2 nanostructures reduces the overlap of CS during adsorption, and thus improves the contact of functional adsorption sites on the surface of MnO2 nanoflakes with heavy metal ions. The results show that the removal efficiency of the nanoadsorbents reaches up to 93% in 3 min for Pb(II). In addition, the maximum adsorption capacity, effects of adsorbent dosage and pH value, and the reusability were investigated. The kinetic process and adsorption isotherm fit well with the pseudo-second-order model and Langmuir model, respectively. These findings provide a potential strategy to address the overlap issue of some common nanoadsorbents.
Heavy metal ions in water are severely harmful to human health (Repo et al. 2013; Carpenter et al. 2015; Chiavola et al. 2015; Gupta et al. 2016). For example, it has been confirmed that Pb(II) increases the risk of cancer and intelligence defects (Clausen et al. 2011; Bhatluri et al. 2015; Du et al. 2016; Jiang et al. 2016). The efficient removal of heavy metal ions from aqueous solution is significant. In recent years, among many other approaches (such as membrane filtration and ion exchange), adsorption of pollutants from water by utilizing adsorbents has attracted broad attention because of its low cost and easy operation, etc. (Chen et al. 2013; Yin et al. 2013; Raychoudhury et al. 2015; Rodriguez-Flores et al. 2015). However, the adsorption rate and capacity of many adsorbents are as yet non-ideal, which limits the applications of adsorbents, especially for the purification of drinking water. Recently, nanomaterials with large surface area and functional groups which can capture pollutants specifically have been considered promising adsorbents (Liu et al. 2015; Sharma et al. 2015; Adeleye et al. 2016).
Chitosan (CS), which is derived from the deacetylation of chitin, possesses many fascinating features including abundance, multifunctionality, and non-toxicity (Sowasod et al. 2013; An et al. 2015; Sun et al. 2016). CS is considered to be a promising material for adsorption of heavy metal ions from water due to its special molecular structure, which contains a lot of amino and hydroxyl groups. However, CS aggregates in solution easily, which limits its potential applications. Despite many significant efforts focused on the cross-linking and modification of CS, reducing aggregation and improving the effective surface area exposed to pollutants remain a great challenge.
In a solution environment without sufficient stirring, CS nanoparticles (NPs) would aggregate and overlap due to the requirement of automatically reducing surface energy (Anson et al. 2004; Michiardi et al. 2007), resulting in the adsorption sites located in deep positions being covered by the top layer. In this condition, many adsorption sites can hardly be effective at contacting and adsorbing surrounding heavy metal ions. Here, we present a promising strategy to address the overlap issue of CS during adsorption. In our investigation, CS NPs are loaded onto a flower-like MnO2 nanostructure which is chemically stable and monodispersive. The flower-like MnO2 with its branched morphology keeps sufficient spaces between adjacent MnO2 materials, providing good conditions to ensure CS NPs located on each petal are well exposed to the surroundings. This adsorbent possesses the following features: (1) the aggregation of CS is effectively reduced due to the support of the three-dimensional (3D) flower-like MnO2, enabling the functional groups of CS to be exposed to heavy metal ions (Pb(II) was employed here); (2) the size of CS NPs is small, which means the CS possesses high surface area and high reactivity for chemically binding with Pb(II); (3) the structure of the adsorbents is stable because of the chemical stability of MnO2, and thus the adsorbents can remove heavy metal ions in neutral drinking water.
The preparation procedure for MnO2 nanoflakes is similar to that described in a previous report (Zhang et al. 2013) with some modifications. Typically, 2 mmol of MnSO4·H2O and 2 mmol K2S2O8 were dissolved into 35 mL of de-ionized water under stirring for 30 min. The obtained solution was transferred to a Teflon-lined stainless steel autoclave, which was subsequently sealed and heated in an oven at 150 °C for 30 min. After naturally cooling down, the black precipitates were collected and washed alternately with water and ethanol. At last, the as-prepared samples were dried in a vacuum oven at 60 °C for 5 h.
In order to modify the MnO2 nanoflakes with CS NPs, the following steps were conducted. First, 0.1 mL of CH3COOH was added into 40 mL of de-ionized water to form a homogeneous solution. Then, 0.04 g of CS was added into the solution under stirring until clarification. After that, 0.05 g of MnO2 nanoflakes was put into the solution under stirring for 30 min. Na2CO3 was used for adjusting the pH value to 8 to nucleate the CS particles. Finally, after stirring for an additional 30 min, the samples were collected and washed with water, and dried in a vacuum oven at 60 °C for 3 h.
The samples were characterized by using a Philips X’Pert Pro X-ray diffractometer (XRD), an FEI Sirion 200 field emission scanning electronic microscope (FESEM), and a JEOL JEM-2010 transmission electron microscope (TEM). Fourier transform infrared (FTIR) spectra were recorded on a Shimadzu IR-440 spectrometer. Surface area was measured on a Coulter Omnisorp 100CX Brunauer–Emmett–Teller (BET).
Pb(II) was used as the target heavy metal ion for adsorption. In a typical procedure, 20 mL Pb(II) aqueous solutions, prepared by dissolving Pb(NO3)2 (Sigma-Aldrich Corp. without further purification) into de-ionized water at different concentrations ranging from ∼10 to 400 mg L−1, were dispersed with 20 mg of the as-obtained adsorbents. All adsorption was conducted in a vibration water bath at 25 °C. After a certain period of adsorption time, the solutions were separated and collected by centrifuge. The remaining concentrations of Pb(II) were measured by a Jarrell-Ash ICAP-9000 inductively coupled plasma. For the investigation of the adsorption effect of adsorbent dosage, series amounts of adsorbent ranging from 0.1 to 2 g L−1 were used, while the initial concentration of Pb(II) and adsorption time were 20 mg L−1 and 2 h for each adsorption, respectively. In pH-dependent adsorptions (initial concentration of Pb(II): 10 mg L−1; adsorption time: 3 h), pH values ranged from 2 to 6, and were adjusted by 0.5 M HCl or NaOH solutions.
RESULTS AND DISCUSSION
In summary, we present an effective strategy to utilize CS NPs for fast adsorption of Pb(II) from aqueous solution. Dense CS NPs are modified on monodispersive flower-like MnO2 nanoflakes. The overlap of CS NPs can be significantly reduced, thereby improving the exposure of adsorption sites to heavy metal ions in solution. The adsorbents exhibit a high removal efficiency of about 93% in 3 min for Pb(II), and a maximum adsorption capability around 102.5 mg g−1. The results show that the adsorption efficiency increases depending on the increase of adsorbent dosage; in the range from 2 to 6, a high pH value is favorable for high adsorption efficiency; and the removal percentage remains above 91% after four regeneration cycles. It is expected that the presented nanoadsorbents could be a promising candidate for water purification; and the strategy of loading NP onto flower-like architectures would also stimulate some new studies about high-performance nanoadsorbents.
This work was financially supported by the State Key Project of Fundamental Research for Nanoscience and Nanotechnology (2011CB933700), and the National Natural Science Foundation of China (51002157, 21277146, 61071054, and 21177131).