¹ Water, Applied and Environmental Chemistry Group, University of Córdoba, Cra 6 N° 76-103 Monteria, Colombia

² Department of Analytical Chemistry, Chemistry Faculty, Mexico's National Autonomous University, C.P. 04510 México

Abstract

It was evaluated the technical viability of mercury removal by electrodeposition from vegetal biomass samples obtained from mining zones which had 10±0.3 μg Hg g^-1. Each sample was treated by mixed acid to destroy the organic matter and liberate the metal in its inorganic form for the later removal of Hg by means of a cell of electrolysis with a rotary electrode of copper as cathode. Mercury concentration was determined by Cold Vapor Atomic Absorption Spectrophotometry (CVAAS). Response surface methodology (RSM) was applied to evaluate the simple and combined effects of three independent parameters (voltage, time and concentration) on the removal efficiency and optimizing the operating conditions. Analysis of variance showed a high coefficient of determination (r² = 0.925) indicating that the second order regression model explains 92.5 % of the variability in results. The maximum efficiency of removal (91.2 %) predicted by the model was found for the initial concentration of 1.0 μg mL^-1, 66.6 mins and 34.3 V. Model Validation was carried out under the following conditions: 1.0 μg mL^-1, 60 mins and 30 V, which are close to the maximum efficiency with a removal percentage of 87.1 %.

Keywords: mercury, biomass, electrodeposition, Response Surface Methodology (RSM).

Introduction

Mercury is one of the metals with high impact on the ecosystems and has been the object of innumerable studies achieved by investigators from several areas [1]. Mercury (Hg), as any other chemical element, cannot be either chemistry or biologically degraded [2]. This metal is released into the environment by both natural and anthropogenic sources; as a global pollutant it is ubiquitous in the planet; in addition mercury bioaccumulation and biomagnification occurs across trophic chain due to its persistence, leading to negative impacts on ecosystems and public health [3]. Plants have different natural properties what are ideal to be used for cleaning up air, soils and water bodies contaminated with heavy metals. Nowadays these properties are used to implement a known green technology system which is named phytoremediation [4]. In recent years, several studies have demonstrated the mercury removal potential from soil and water through plants [5-7]. Nevertheless, metals are not metabolized by plants but accumulate into their biomass which causes another environmental problem [8]. Literature mentions that biomass can be confined into sanitary fillings or used like compost [9-10], fact that is not advisable at all due to the high chance of release of that pollutant into the environment, what is subjected to the changes in environmental conditions. Drying, compaction and later incineration of the vegetal biomass have been suggested for recovering metals of high commercial value [11]. Nevertheless studies about this topic are still scarce [12]. An up to date review of the existing literature proved that the only study of electrochemical removal of metals from vegetal biomass with lead removal efficiencies between 93 and 95 % [13]; according to the results the application of electrochemical methods focused on removal or recovery of metals from contaminated vegetal biomass becomes necessary. The aim of this work was to evaluate the electrodeposition process of mercury removal from solutions obtained of contaminated vegetal biomass to different conditions of concentration, potential and time, optimizing the conditions of maximum removal efficiency by response surface methodology (RSM).

Materials and methods

Treatment of vegetal biomass

This solution was used later in the electrodeposition process. Hg concentrations were determined by Cold Vapor Atomic Absorption Spectrophotometry methods, adapted for analysis of plant samples and water [15-16]. Samples were analyzed in triplicate and concentrations were reported as μg Hg mL^-1 (liquid matrix) and μg Hg g^-1 (solid matrix).

Experimental design

Levels of every factor were evaluated in triplicate, 81 essays in the whole process. With the purpose of evaluating matrix interferences for any other metal, independent electrodeposition essays were realized with synthetic solution of mercury 3.0 μg mL^-1 to potentials of 10, 30 and 50 Volts, simulating the electrolytic media by means of mixed acid addition H₂SO₄ - HNO₃ (7:3) as made in the digestion process.

The percentage of mercury removal was evaluated by determining the decreasing of the initial mercury concentration into the biomass solution digested as a function of the electrodeposition time. The equation to calculate the removal efficiency in the experiments was:

where Cf : final Hg concentration ; Ci : initial Hg concentration.

Electrodeposition cell

The volume used for every essay was 250 mL of digested biomass, which were added into an electrodeposition cell made up by a cylindrical plastic container with the following dimensions: 5.0 cm (height) per 8.4 cm (diameter), provided with a cylindrical rotary copper electrode (cathode) to 1600 RPM to provide agitation to the system, against to a steel electrode (anode) connected to a DC power supply -PHYWE 0-50V. Dimensions of the electrodes were: 15 cm (length) per 2.0 cm (diameter). Distance between electrodes was 6.0 cm. Entire submerged surface of every electrode was 31.4 cm².

Results and discussion

Selection of the electrode material

In this study copper was used as cathode for carrying out the electrodeposition process and recovering the solved mercury into the solution of digested biomass, since this element can form an amalgam with mercury by reduction of Hg²⁺ to Hg⁰ [18], due to the ion migration from the solution and its linkage to the electrode surface by electro reduction when sufficient potential difference is applied. The anode electrode material was the naturally inert or steady steel, which transfers and receives electrons in the solution to generate the electrolysis process [19]. When a cylindrical rotary electrode is used as a cathodic cell, it is named cell of rotary electrodes [20], in which the electrode rotation causes a permanent homogeneity, a controlled convective regime into the solution, more uniform electrode coverings and suitable ionic transport; all of that favors the changes from the cathode zone with the rest of the electrolyte, improving this way the metal deposition [21].

Hg removal function of time, potential and concentration

where y is the response variable of the removal efficiency, b₀ is a constant, b₁, b₂ and b₃ are the regression coefficients of linear effects, b₁₁, b₂₂ and b₃₃ are the second grade coefficients, and b₁₂, b₁₃ and b₂₃ are the interaction coefficients.

As recommended in the literature, the coefficient of determination (r²) should be at least 75 % [17] to consider the possibility of using RSM. In this study case a high coefficient of determination (r² = 0.925) was obtained, which implies that 93 % of the variations in Hg removal efficiency are explained through independent variables, and 7 % of variations cannot be explained by the model.

Significant differences were not observed (p < 0.05) when a point near to the maximum response is compared to the corresponding experimental value what confirms that RSM can be used to optimize the process parameters.

Every color stripe contains an infinite number of the next kind of combinations: concentration-time, concentration-voltage, voltage-time, for which ones the average recovery predicted is in a certain range; for example, it is possible to observe that the stripe indicated with the letter P for every contour plot contains the combinations of the variables of interest, which can estimate the average recoveries between 80 %-100 %, 90 %-95 % and higher than 80 %.

In this study it is observed that the best results are in the red region, where the interaction of the factors leads to results between 80 and 100 %.

Conclusion

Mercury electrodeposition by means of copper electrode as cathode is a technically viable alternative for the treatment of contaminated biomass. This study demonstrated that the response surface methodology is an adequate method to optimize the operation conditions and to maximize the mercury removal from digested vegetable biomass. The analysis of variance showed a high coefficient of determination (r² = 0.925) which guarantees a satisfactory adjustment of the second order regression model to the experimental information. The response surface and contour plots can be used for locating an optimum point. The applied electrochemical treatment allowed the removal of Hg in 87.1% under conditions of optimal electrodeposition: 30 V, 60 minutes and concentration of 1.0 μg mL^-1.

References