(212d) Biocomposite Material from Fique and Iron Nanoparticles As Adsorbent for Mercury Removal from Aqueous Solutions: Kinetic and Equilibrium Studies

The wide range of contaminants in surface and groundwater has become a critical issue worldwide, due to population growth, rapid industrialization and development of long-term drought. Persistent pollutants in wastewater include heavy metals, inorganic and organic molecules, and many other complex compounds. As a result, the number of available water resources in nature to sustain life has been decreasing over time, becoming one of the biggest problems nowadays, because of the diversity of pollutants and the rapid depletion of available sources. Therefore, there is the need for controlling the harmful effects of pollutants and to improve the conditions of human life, for example according to WHO, mercury is one of ten products or groups of chemicals that pose special public health problem due to its high contamination potential and its widespread use in not regulated industries [1].

Mainly, mercury has become an important contamination vector in areas were mining of noble metals is active (especially in illegal extraction operations), one of the major source of mercury contamination is the gold extraction process [2]. This issue is of special interest in Colombia, where illegal mineral extraction uses extensive quantities of mercury, affecting atmospheric and water quality [3]. This problem is particularly alarming in the small-scale gold mining process (usually artisanal performed), where one kilogram of gold obtained released into the environment three to five kilograms of mercury [4]. Increasing the statistics of Colombians health problems related to neurological symptoms, decreased motor skills, impaired hearing and vision, among others [2]. Although gold mining is an activity with significant importance in Colombian economy (producing 30 tons per year), environmental control is poorly structured, especially in the areas where handmade extraction is practiced; creating at the same time environmental liabilities, turning this practice into an unsustainable process [4] [5].

Mercury separation by adsorption, has been an alternative that takes advantage of leverage the physical interactions between the solid surface and adsorbate. Different materials can be used as adsorbents, depending on the application, price, mechanical and chemical resistance, abundance, easiness to use, easy regeneration, adsorption capacity and surface area. In this works, a new economic and efficient alternative to remove mercury (II) from aqueous effluents has been investigated. A biocomposite material using iron nanoparticles anchored onto Fique fiber (lignocellulosic biomass), were developed as adsorbent.

In the adsorbent synthesis raw fique fiber was pretreated during 60 minutes under sonication with the aim to increases the crystalline cellulose and clean fiber, then the pretreated fiber was functionalized by cationization treatment under acid and basic conditions. In order to obtain iron nanoparticles anchored into the fique fiber, an impregnation method was implemented, the functionalized fiber was submerged in chloride ferric changing the time on one, two and three days. After, the impregnated fiber was soaked in a reducing solution of 1M NaBH₄ (Merck Millipore®) per 10 minutes, holding the molar ratio 2:1 (BH₄ˉ / Fe⁺³), finally the adsorbent was dried at room temperature.This material was characterized by Atomic Absorption Spectroscopy (AAS), X-Ray Diffraction (XRD), Fourier-Transform Infrared Spectroscopy (FTIR) and Scanning electron microscope (SEM).

Preliminary experiments were performed to determine the amount of adsorbent material to use in the adsorption (400, 800, 1000 and 1500 mg). The adsorption experiments were arranged in a jacketed glass batch reactor, with continuous agitation (240rpm), using 800 mg of adsorbent in contact with 150 mL of mercury solutions. Each run lasted 30 minutes. All experiments were performed at 20°C and 551mmHg. Every 5 minutes an aliquot was taken and analyzed for mercury (II) using vapor atomic absorption spectrometry (CVAAS). A Box-Behnken experimental design evaluated the effect of pH (4-10), initial concentration of mercury ions [Hg²⁺] (1-14 ppm), and load of iron in the adsorbent material (10.9-14.9 Fe wt.%), in the removal of mercury (II).

The equilibrium was reached after 15 minutes, with a maximum value of 92.43% mercury removal of mercury (II) ions from aqueous solution, under following conditions: pH: 4, mercury initial concentration:10 ppm and 10.9%wt load of iron in the material. The adsorption process was found to be pH dependent, due to pH has no apparent effect on adsorption in the range of pH 4 to 7. Nevertheless, for pH greater than 8, the adsorption capacity increases as the pH value also increase. The thermodynamic study confirmed that the adsorption was spontaneous and exothermic in nature.

In addition, eight models were adjusted to understand how the adsorption process was carried out, these models included: adsorption diffusion models, adsorption reaction models, chemisorptions kinetics model and adsorption isotherms. It was evidenced the chemisorption mechanism predominates respect to physisorption, Langmuir, Freundlich and Tempkin isotherms were applied to describe equilibrium data. However, they failed to properly describe most of the experimental isotherms obtained. It was found that the kinetics of mercury adsorption onto the surface of the biocomposite is described by the pseudo-second order model supporting chemisorption (chemical reaction) as rate controlling mechanism which indicates that the adsorption process is irreversible. Thus, biocomposite was found to be superior adsorbent compared to other adsorbents for mercury (II) ions from aqueous solutions.