Neoteric Material Based on Renewable Resources for Metal-Contaminated Waters ^†

Abstract

A continuous increase of environmental pollution has been recorded worldwide, during recent decades, as a result of industrialization and urbanization. In particular, metal release in the environmental media may threaten human health, due to their persistence and accumulation in the food chain. We report here the functionalization of chitosan with poly(benzofurane-co-arylacetic) acid, which is a new material with the ability of complex metals from contaminated water. The synthesized polymer was structurally investigated by scanning electron microscopy (SEM), thermogravimetric analysis (TGA), X-ray photon electron microscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR), while heavy metals were determined by atomic absorption spectrometry. Different isotherms and kinetic models were used to describe the absorption equilibrium and the behavior of the material, based on the initial pollutant concentration and contact time. The results are pointing out that such natural materials can be easily synthesized, at low costs, thus offering attractive solutions for wastewater treatment.

1. Introduction

Environmental pollution due to uncontrolled heavy metals release may threaten human health and biota. Around 25% of the total Romanian water bodies are classified as having moderate, poor, and bad quality (3rd, 4th and 5th quality class), and more than 900,000 ha of brownfields (contaminated and potentially contaminated soils) are identified on a national scale (which further become pollution sources for surface and groundwater) [1,2]. Recently, researchers have been focusing on developing new materials through "green chemistry methods” to remove pollutants from the environment. Nature provides a wide range of materials with different functional groups, which serve as a source of inspiration for scientists working on new materials [3].

In this study, we report the synthesis and characterization of a new material (PAAA-CHIT) based on renewable resources—poly(benzofuran-co-arylacetic acid) (PBAAA) functionalized with chitosan (CHIT). The biodegradable material obtained was further used for heavy metal absorption from stock solutions and contaminated water samples collected from Roșia Montană Mining Area (Romania). Human exposure doses were estimated to assess potential health risks associated with metal contaminated waters.

2. Materials and Methods

PBAAA was synthesized by closely following a previously reported procedure [4]. In brief, the neoteric materials were prepared by modifying PBAAA with chitosan. All reagents used were of analytical grade. The newly synthesized material was analyzed by Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and X-ray photon electron microscopy (XPS), while metal concentrations were determined by atomic absorption spectrometry (AAS).

Absorption assays were applied on heavy metal stock solutions of different concentrations (10, 20, 40, 60, and 100 mg/L). A mixture of 40 mL metal solution and 800 mg absorbent material (PAAA-CHIT) was stirred for 24 h at 600 rpm under normal atmospheric conditions. Samples were then filtered off and analyzed by FAAS. The removal efficiencies and sorption capacities were calculated based on Equations (1) and (2). Experimental data on absorption were fitted on Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Khan equilibrium isotherm models and pseudo-first order, pseudo-second order and Weber–Morris intra-particle diffusion kinetic models [5,6,7,8,9,10,11,12].

$$R (\%)=\frac{Ci-Cf}{Ci}\times 100$$

(1)

$$q \left(\mathrm{mg}/\mathrm{g}\right)=\frac{\left(C_i-Cf\right)\times V}{w}$$

(2)

where: R is the removal efficiency (%), Ci is the initial concentration (mg/L), Cf is the final concentration (mg/L), q is the sorption capacity (mg/g), V is the volume of solution (L) and w is the amount of material used (g).

Human exposure and potentially associated health risks were also estimated in relation to the exposure via metal contaminated waters from the Roșia Montană area [13,14,15].

3. Results and Discussion

The new material was easily prepared and separated. Significant changes were recorded on the heavy metal concentrations remaining after performing absorption experiments with PAAA-CHIT. The metal absorption efficiencies were as follows: Zn > Cu > Fe > Pb > Cd > Cr > Ni > Mn. Good results were also obtained on the materials’ efficiency and selectivity in retaining metal ions, in water samples collected from mining areas.

The estimated hazard quotients and indices, which may indicate a potential health risk related to the exposure to contaminants in the environmental media, were below the safe level of 1, showing that it is not likely that the metal contaminants in the water samples will pose a risk on potentially exposed population groups.

4. Conclusions

A new type of material based on renewable resources was developed through an accessible non-catalytic synthesis method. The new material exhibited good absorption properties, which makes it suitable for applications in wastewater treatment.