There is a wide variety of substances that pollute the environment, including metals that have achieved high levels of concentration in many areas, creating a toxic effect on living organisms [1]. In recent years, a new scientific field was developed to reduce the cost of removal treatments of metals in solution: biosorption [2]. Early works showed that some biological materials can extract metal contaminants from water at very low cost, taking advantage of their selective physicochemical properties. Additional benefits of these treatments are its public acceptance and the possibility to be used with high performance under optimal and controlled conditions. New technologies for the recovery of these metals are in development [3].

Among the biopolymers with capture capability that can be used as biomass, the Exo-Polysaccharides (EPS) are biodegradable and with an interesting low-cost production [4]. There are a great number of microbial EPS that can interact with metal ions [5], [6]. These substances have a complex superficial chemistry with various functional groups, which together show different trends to join with other substances or ions [7], [8].

The classic characterization techniques for these systems are complex, laborious, with very high detection limits and low sensitivity. In addition, a considerable amount of sample to be analyzed is required. This has been a limitation to realize fast and precise experiences of transport in both, static and dynamic conditions. X-ray Micro-Fluorescence (XRµF) is an energy dispersive spectrometric technique with high resolution properties that allows studying dynamic systems. “In situ” XRµF was selected as the no-invasive technique to determine local concentrations in this study [9], [10].

Usually, to the design of a Plug Flow Reactor (PFR), the reaction rate is determined by tests on a Continuous Stirred-Tank Reactor (CSTR), these generate toxic effluents and also increase the cost of the design. In this work, an empirical expression (called “slip function”) in terms of the average concentration of the contaminant, was developed through the study of the transport behavior of Cr^VI into solutions of xanthan gum, as a model system. Chromium, one of the most common heavy metals contaminants [11] and, xanthan, a biodegradable and low-cost EPS, were chosen for this study [12].

The presentation of a more ecological way to design PFR for treatment of aqueous industrial effluents, and the proposal of the use of biodegradable and recyclable polymers of low cost, capable of retaining contaminants is the purpose of this work.

Experimental slip functions were used with laboratory PFR experiments planned in similar conditions, to obtain useful dimensionless parameters for the industrial design.

At the present time, the study of green chemical solutions for turnover times and subsequent recycling or reuse of both the polymeric material and the recovered metal are undertaken.

X-ray Microfluorescence. The experimental data sets from SRµXRF experiences may be shown as a series of curves of concentration versus time for each position. These sets were obtained from the diffusion of chromium through different concentrations of xanthan gum solutions. As example, Figure 4 shows the data set corresponding to the diffusion of Cr^VI through a 7,000 mgL^-1 xanthan gum solution.

Figure 4.

Cr^VI concentration at different times and positions, obtained during a SRµXRF scanner of the sample cell with 7,000 mgL^-1 xanthan solution

Slip funtions (f_s) determination. Measuring the change in the concentration ΔC of a ion in a point x at different times t, the increasing in mol per unit volume and time at that point could be calculated. This can be interpreted as the molar rate per unit volume at which these ions cross through that point at that time. A plot of ΔC/Δt as a function of the average concentrations C_m for that interval Δt in the same point x, shows the local variation of molar average rates per unit volume. Data could be fitted to a quadratic function of the type of (eq. 1):

where α_s and β_s are empirical parameters depending on the ion range of concentration and polymer concentration. The values for Cr^VI in 7,000 mgL^-1 xantan obtained by fitting (R² = 0.976) are shown in Table 1.

Obtained by this way, the called slip function f_s is related with the accumulation rate of the ion in this system (Figure 5). f_s necessarily vary with the nature of the ion, the type and concentration of the polymer and the temperature. Conceptually, f_s is a measure of the ability of the ion to stay or be retained in that system.

Figure 5.

Slip functions obtained from chromium moving in xanthan solutions calculated from experimental data

Higher ion rates are in dilute systems and lower in concentrated systems. However an anomaly was observed: The f_s for xanthan 7,000 mgL^-1 falls below that of 8,000 mgL^-1. This can be explained in terms of the number of available active sites, due to easier access within a less concentrated solution. That is, despite there being a greater number of active sites as there are more macromolecules, not all of these sites are available because this high concentration impedes access. And then at 10,000 mgL^-1, a gel is formed that retains ions also by steric effects. The objective of this work is to use solutions rather than gels, by the advantages offered as its recyclability and the lower operating costs of the equipment involved (eg. lower pressure of work). The optimal range for our design will then be about 7,000 mgL^-1 of xanthan, that retains the advantages of being a solution and presents a maximum of active sites.

Experiences in the PFR. In a plug flow reactor at steady state, the ion concentration in the fluid C varies with the position in the direction of flow F, due to retention. The mass balance (eq. 2) refers to a differential volume element dV moving along the reactor with a given retention rate ν_R:

In this work the experimental slip functions are employed as the retention rate functions as is shown in (eq. 3):

The ion residence time τ_s using the slip functions, in a reactor of volume V was calculated as in shown in (eq. 4):

Using the ratio of the experimental replacement time t_re and the residence time, fixed in this experience at 60 min (Figure 6), a dimensionless parameter, θ_s, is generated (eq. 5). Experimental value is shown in Table 1.

Figure 6.

Output Cr concentration from the laboratory PFR reactor as a function of the time

To confirm the independence of θ_s of the conditions of this particular experience, several similar experiments were conducted with different residence times. It was noted that θ_s value remains constant by doubling or by taking half the value of the volumetric flow through the reactor. Thus, a design parameter was obtained that indicates the number of residence times that any reactor may comply before that its filler should be replaced, for the given operating conditions (range of Cr^VI concentrations, xanthan concentration and temperature).

To make the change of scale to the industrial reactor, other dimensionless parameter was generated as the ratio of the replacement time value and the value of the residence time calculated (eq. 6). Experimental value is shown in Table 1.

The values found for this dimensionless number in experiences with different systems indicate that this is characteristic of the nature of the polymer, but does not depend on its concentration neither of the nature or concentration of the ion.

This parameter is used in the design as follows: τ_s is calculated by using the slip function integrated between the concentrations of interest and with k_s, the replacement time is obtained. With this value, the θ_s parameter is then used to obtain the residence time of the industrial reactor and to calculate its volume for a known flow. The volume thus determined maximizes the design with regard to the efficiency of the filler that was tested in a microcell, with only a few milliliters of contaminant solution.

It has been shown that the slip functions can be used as retention rate equation in reactor design. The defined dimensionless parameters (ϴ_s, k_s) link between experimental values obtained in a laboratory reactor t_re, and the values calculated using the slip functions τ_s. It has been demonstrated from these experiments that the obtained values of θ_s can be used to estimate the time of replacement of the industrial prototype.

The presented methodology involves fewer quantities of drugs, and is faster than classical. Experiments with the laboratory PFR were essential to obtain the parameters required for the design, but, as with any engineering problem, the application will encounter compromise situations that should be resolved, involving costs and operation.

Heavy metals and the biopolymer can be recycled. When the polymer is out of stock, it can be easily cleaned and eliminated by biodegradation in digesters, thus closing the cycle of providing energy and fertilizer use.

This design method, which was described for the retention of contaminants by polymer solutions, could be applied to any other type of retention media.

In the near future when the constraints imposed by the environment were intensified and enforced, surely there will be more than one application for this kind of environmentally friendly industrial processes [18]-[20].