Supported carbon dots serve as high-performance adsorbent for the retention of trace cadmium

Highlights

• Supported carbon dots were prepared by modifying hydrosoluble carbon dots on a microcarrier as a novel SPE adsorbent.

• The composites merged abundant active sites with great mechanical strength, which was suitable for cadmium preconcentration.

• The composites exhibit fast adsorption/desorption dynamics and good adsorption stability against foreign metal species.

Abstract

Carbon dots were prepared via a one-pot hydrothermal route, and a new solid-phase extraction (SPE) adsorbent was developed by immobilizing the carbon dots on the microcarrier cytopore, shortly termed as C-dots@cytopore. The C-dots@cytopore composites were characterized by means of FT-IR, SEM, XPS and fluorescence spectrometry. The performance of the composites for the adsorption of heavy metals was thoroughly evaluated by using cadmium as a model. The binding of cadmium on C-dots@cytopore fits Langmuir adsorption, and the adsorption dynamic follows pseudo-second-order adsorption kinetics model. The binding of cadmium was pH-dependent, with a maximum adsorption capacity of 2420 μg g⁻¹ obtained at pH 4–7. A novel separation and preconcentration procedure was thus developed for trace cadmium using the C-dots@cytopore composites as SPE sorbent. The retained cadmium could be readily eluted and recovered by a 0.1 mol L⁻¹ HNO₃ solution and further quantified with graphite furnace atomic absorption spectrometry (GFAAS). With a sample volume of 1.0 mL, an enrichment factor of 17.85 was obtained with a detection limit of 1.8 ng L⁻¹ and a RSD value of 2.6% at 0.1 μg L⁻¹ (n = 9). The procedure was further validated by analyzing cadmium in certified reference materials and a series of environmental water samples.

Introduction

Cadmium is a highly toxic metal element which gives rise to serious environmental pollution problems [1]. With the rapid development of industry, large amount of industrial wastes were discharged into the environment without treatment, causing excessive cadmium accumulation in the natural world. Cadmium can easily enter into the human body through food chain but it is generally very hard to be metabolized out of the system [2], [3]. Even at very low concentration levels, it could cause serious damages to human organs, e.g., anemia, abdominal pain, kidney and skeletal systems, in addition to its carcinogenic feature [4], [5], [6]. Thus the accurate quantification of cadmium and the timely warning of its risk, especially at ultra-trace level in environmental samples, are of significant importance.

Due to the high complexity of the sample matrixes and the low level of cadmium in real samples, an effective and efficient separation-preconcentration step prior to its analysis is generally highly desired. At the present, common sample pretreatment approaches including cloud point extraction [7], liquid-liquid microextraction [8], liquid-liquid extraction [9] and solid-phase extraction (SPE) [10] have been extensively applied in the separation and preconcentration of various trace metal elements. Compared with other preconcentration processes, SPE technique is most widely employed on account of the high extraction efficiency, less solvent consumption and relatively convenient operation [11], [12]. Moreover, diverse choices of solid-phase adsorbents and ease of surface modification of the materials provided broad spaces for the development of SPE of heavy metals.

A variety of novel materials, e.g., carbonaceous material [13], molecular or ion-imprinted polymers [14], [15], mesoporous material [16] and magnetic material [17], have been widely used in solid phase extraction. Among these, carbon nanostructures attract extensive attention relied on their unique physical and chemical properties [18]. At this respect, nanomaterials could be classified as zero-, one-, two- and three-dimensional nanostructures. Nowadays, a large amount of carbon nanophase materials including nanotubes, nanosheets and nanofibers have been successfully applied in the separation and enrichment of the analytes at ultra-trace level [19], [20], [21]. Carbon dots is a type of zero-dimensional nano-sized material [22]. Massive oxygen-containing groups on the surface of carbon dots, e.g., carboxyl and hydroxyl groups, offer sufficient binding sites for multiple metal cations through electrostatic attraction, complexation and other interactions. It is remarkable that different approaches were exploited to regulate the functional groups contained on the carbon dots in order to improve the selectivity towards specific metals [23], [24]. Carbon dots with large surface area provide more active sites with respect to that provided by bulky adsorbent, facilitating fast adsorption and desorption dynamics. Furthermore, benefited from abundant material sources, convenient and diverse fabrication process and stable physical/chemical properties, carbon dots are qualified as a good alternative for the SPE of metal cations [25], [26]. However, it is known that in practice carbon dots are very hard to be separated from the aqueous medium through centrifugation due to their tiny size (in the range of approximately 1–10 nm) and hydrophilic property, which largely limits their application in the field of sample pretreatment.

In the present study, carbon dots were employed as a novel adsorbent for the SPE of trace level of heavy metals. The carbon dots were prepared through a one-pot hydrothermal method and then immobilized onto the surface of the microcarrier cytopore, resulting in a unique solid adsorbent, i.e., C-dots@cytopore composites. The supported carbon dots which merged the mechanical strength together with the micro-size property were proven to be very suitable for the adsorption of cadmium. The effects of various analytical parameters related to the SPE of cadmium, i.e., the acidity of sample solution, the volume and concentration of the eluent as well as interfering effects were studied. The novel composites were successfully applied in the separation and preconcentration of trace level of cadmium in real environmental samples.