Silica conjugated with kraft lignin and its use as a novel ‘green’ sorbent for hazardous metal ions removal

Highlights

• Silica/lignin hybrid material with defined surface and physicochemical properties were developed.

• Advanced silica/lignin material as a novel ‘green’ sorbent for Ni²⁺ and Cd²⁺ ions removal.

• The pseudo second-order and the Langmuir isotherm model were suitable to characterize the process of adsorption.

Abstract

An advanced functional silica/lignin material was obtained and tested as a novel adsorbent of nickel(II) and cadmium(II) ions. The ‘green’ sorbent was obtained chemically from the commercial silica Syloid®244 and kraft lignin. The resulting hybrid material exhibited specific physicochemical and structural properties, which were determined by a number of methods, including Fourier transform infrared spectroscopy, elemental analysis, scanning electron microscopy, electrophoretic light scattering, nitrogen sorption and others. The surface area of the hybrid material was 223 m²/g. The obtained material was tested as an adsorbent of hazardous metals from aqueous solutions, and an analysis was made of parameters influencing the effectiveness of the adsorption process, such as duration, pH of solution and mass of adsorbent. The kinetics of the process were approximated by pseudo-first-order and pseudo-second-order models; the experimental data were found to correspond well to the pseudo-second-order model, with correlation coefficient r² = 0.999. The isotherms of adsorption equilibrium were fitted by the Langmuir and Freundlich models using a nonlinear regression method, and good correspondence of the experimental data to the Langmuir isotherm was obtained.

Introduction

Increased populations and intensive industrial development are bringing about significant increases in the level of pollution of water systems. Among the most toxic pollutants are ions of environmentally harmful metals, including nickel(II) and cadmium(II) ions [1], [2], [3], whose adsorption on a silica/lignin functional hybrid material will be described here. The risk posed by the presence of these metal ions in water bodies is linked to their mobility in aquatic ecosystems and the possibility of accumulation in living tissue. Moreover they cause numerous diseases and disorders in the human body. Nickel(II) ions can cause chronic bronchitis, impaired lung functioning, and even lung cancer, while cadmium(II) ions may lead to pulmonary fibrosis and dyspnea, and have been confirmed to have significant carcinogenic properties. In view of the serious danger resulting from the presence of environmentally harmful metals in water streams, many countries have introduced strict regulations to control levels of water pollution. In the European Union the permissible content of nickel(II) ions is 0.02 mg/L, while that of cadmium(II) ions is 0.005 mg/L [4]. This makes clear the necessity of continuously improving and seeking new methods for purifying aqueous solutions.

Currently known and commonly used methods for removing harmful metal ions include chemical precipitation [5], [6], ion exchange [7], extraction [8], membrane techniques [9], and adsorption [10], [11], [12], [13], [14]. In spite of their satisfactory effectiveness in purifying water, most of these methods involve expensive processes and complex methodology, and in many cases also entail the irreversible use of chemical compounds, which may lead to secondary pollution [15]. A method of great interest for removing hazardous metal ions is adsorption. The functional and economic effectiveness of this method are dependent on the sorbent used [16].

Many scientific institutions are carrying out research to discover and produce effective and economical sorbents of both natural and synthetic origin [17], [18], [19], [20]. Adsorbents of natural origin include biomass materials [16], as well as agricultural waste products and by-products of certain industrial processes, for example in the food, paper, coal and pesticide industries [21], [22], [23]. Biomass has very favorable adsorption properties, and may also serve as a precursor for the production of functional biomaterials [16]. There are also many reports in the literature concerning industrial wastes, which are available locally in significant quantities in many regions of the world. These can be modified or suitably processed to obtain final products with satisfactory sorption properties [24], [25], [26]. Synthetic adsorbents, on the other hand, include certain oxides and co-precipitated oxide systems, hybrid systems and composites [15], [16], [27], [28], [29], [30], [31], [32], [33]. Nanoporous silicas are of particular interest in waste water treatment because their high surface area provides a very large capacity for the adsorption of harmful pollutants [34]. Moreover, reactive groups on silica surface make it ideal for chemical modification in order to obtain sorbent with specific functionality and adjustment of the sorbent selectivity for the capture of target hazardous compounds [34], [35]. The most popular modifying agents for silica are silanes [34], [35], [36], [37]. However, sustainable chemistry and engineering causes development of materials and technologies based on renewable natural polymers. Ongoing technological developments are bringing about increased interest in the use of hybrid systems as effective sorbents. Combining two or more materials at a molecular level may produce more favorable properties than those offered by the precursors [30], [38].

Lignin, as the second most abundant biopolymer, and a by-product of the paper industry, fits perfectly with this trend. Our group has recently found that lignin is an ideal compound for the modification of silica, which may lead to the development of a new generation of hybrid materials. The presence of numerous functional groups in the lignin molecule, especially carboxylic and phenolic groups on the lignin surface, make it ideal for the selective adsorption of harmful metal ions (Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺) [14], [39]. Therefore, the discovery of an effective silica/lignin sorbent will impact on a range of research relating to the utilization of renewable natural materials in developing new materials and technologies for waste water treatment, in line with the principles of sustainable chemistry.

Over recent years a series of research has been carried out with the aim of obtaining a multifunctional biomaterial based on silica combined with the natural polymer lignin. Such a chemical combination of precursors is made possible by the presence of functional oxide groups on the surface of silica, and the high reactivity of lignin [38], [40], [41], [42]. The resulting material combines the favorable properties of both precursors, and thus offers many possibilities of application. The research has made it possible, among other things, to determine the most favorable conditions for the synthesis of SiO₂/lignin hybrid systems so as to obtain the best possible physicochemical and structural properties of the final material [42], [43], [44], [45], which has numerous practical uses. The products obtained have been found to have high surface area, thermal stability and mechanical strength, which means that they can potentially be used as sorbents, but also for example as polymer fillers or electrode materials. Another significant aspect is the non-toxic and biocompatible nature of the resulting organic/inorganic material [46].

In this work we decided to use, for the first time, an innovative functional silica/lignin hybrid material (a ‘green’ sorbent) for the adsorption of Ni²⁺ and Cd²⁺ ions from model solutions. A detailed analysis was made of its action as a sorbent of nickel(II) and cadmium(II) ions from model solutions. It was determined how the process parameters (contact time, pH, and mass of sorbent) influence the effectiveness of Ni²⁺ and Cd²⁺ removal. An important part of the work was the determination of the kinetics of the process using pseudo-first-order and pseudo-second-order models. For the use of the silica/lignin hybrid material as a sorbent of metal ions, it was necessary to determine the adsorption isotherms based on the models of Langmuir and Freundlich.