**3.2.1 Batch experiments**

154 Technical Problems in Patients on Hemodialysis

volumetric fraction correspondent to the secondary micropores. In these charcoals, the secondary micropores and mesopores volumetric fractions have exceeded 37% e 28%,

The activated charcoals of the water treatment in the hemodialysis Center (A and B) have presented secondary micropores and mesopores volumetric portions of 0.15 to 0.38 cm3/g (44.1 to 69.1%) and of 0.01 to 0.09 cm3/g (2.6 to 16.4%), respectively. These charcoals have presented secondary micropores portions higher than those found by Pendleton et al. (2001), however, they have also presented very low mesopores volumetric fractions, around 8% in average, which characterizes microporous charcoals. Nevertheless, those commercial activated charcoals obtained as reference by the authors, have presented secondary micropores volumetric fractions and mesopores of 59.2 to 63.6% and of 34.5 to 41.8%, respectively, which

A visualization of the pores size distribution can be obtained from the distribution function calculated by the HK and BJH (Figure 3) methods. The (A), (B) and (C) charcoals have shown a distribution function HK/BJH, with dW/dLo of 0.04; 0.07 and 0.06 cm3/nm/g consisting of pores average diameters of 3.81, 3.80 and 3.28 nm, respectively. Due to the low adsorption capacity of the D charcoal, it was not possible to obtain its distribution of the pores sizes. For the E charcoal, the distribution function dW/dLo has presented two peaks with 0.02 and 0.01 cm3/nm/g a 2.53 and 4.15 nm in pores average diameter; while the AC-B-F has presented dW/dLo ≈ 0.06 cm3/nm/g to an average pore diameter of 2.7 nm. In contrast with these results, the standards AC-R-G and AC-R-H have shown a distribution function a little higher: (AC-R-G) dW/dLo ≈ 0.3 cm3/nm/g with 1.61 nm of pore average diameter, and AC-R-H has presented two peaks with dW/dLo ≈ 0.17 and 0.08 cm3/nm/g

respectively, favoring the adsorption of microcystin in 200.0 g/mg.

can be characterized as a good indicator for the AC for the treatment water use.

with pore average diameter of approximately 1.71 and 3.00 nm, respectively.

 AC-A-A AC-A-B AC-A-C

 (a) (b) Fig. 3. Function distribution HK/BJH: (■) AC-A-A; (□) AC-A-B; (∆) AC-A-C; () AC-B-E;

According to Lanaras et al. (1991) the MCYST-LR when dissolved in water, has a solvated volume of 2.63 nm, length of 1.9nm, height of 1.5nm, thickness of 1.1nm, and solvated area of 1.8 nm2. The [D-Leu1]MCYST-LR has an amino acid residue, D-Leucine, which has replaced the amino acid residue, D-Alanine, of MCYST-LR, that confers the [D-Leu1]MCYST-LR more hydrophobicity than the MCYST-LR. But, these differences between their chemical structures are not so significant regarding their heights, solvated volume and transversal section area, enabling that approximate dimensions of the first MCYST-LR might

Function distribution dV/dLo / cm3/nm/g

0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40

2 3 4 5 6 7 8 9 10

Log L0 , nm  AC-B-E AC-B-F AC-R-G AC-R-H

2 3 4 5 6 7 8 9 10

Log Lo / nm

() AC-B-F; (Ө) AC-R-G; (О) AC-R-H.

0,00 0,02 0,04 0,06 0,08 0,10 0,12

Function Distribution dV/dLo cm3/nm/g

Knowing the adsorption equilibrium represents the first step in investigating the possibilities for using an adsorbent in a determined separation process. Besides, additional information regarding the distribution of the sizes of the activated charcoal pores can be obtained by comparing the adsorption characteristics of adsorbate by taking those obtained from adsorption data in gaseous phase.

Methylene blue (MB) has been widely used as an adsorbate to estimate the adsorption capacity of CA from continuous in fixed bed or batch experiments (Kumar & Sivanesan, 2006; Macedo et al., 2006; Zhang et al., 2006). Studies on MB adsorption equilibrium in activated charcoal can provide important information about the selectivity of these adsorbents regarding this molecule, given that the MB is accessible to the charcoal pores with an inner diameter greater than 1.5 nm, being important for the characterization of the secondary micropores (0.8 <di <2.0 nm) and mesopores (2 nm < di < 50 nm) mainly, besides being a model compost used for predicting the adsorption of organic contaminants found in industrial effluents such as textile dye and microcystines (Barton, 1987; Baçaoui et al., 2001).

According to the JIS norm (1994), the Methylene Blue Index is operationally defined as the adsorbed amount of that molecule when its residual concentration in liquid phase after equilibrium is of 0.24 mg/L. This adsorption capacity was obtained from an equilibrium isotherm where the experimental data were adjusted to Freundlich's adsorption model. The correlation coefficients for linear regression from the adjustment of the experimental data to the respective linearized model along with its empirical parameters, K and 1/n, besides the charcoals' Methylene Blue Index (MBI), are displayed on Table 3.


\*This charcoal has a very low adsorption capacity (close to zero) for methylene blue, which makes it impossible to calculate the MBI.

Table 3. Freundlich's model adsorption parameters.

Specifications of the Quality of Granulated Activated Charcoal

**3.2.2 Experiments in fixed bed column** 

accessible to adsorption, and the adsorbate.

shown in Figure 5 denominated breakthrough curves.

and AC-R-H (Table 4).

cm3/g).

Used in Water Systems Treatment in Hemodialysis Centers in Brazil 157

and 3.7% (AC-A-B). Still in the equilibrium, toxin adsorption in the charcoals of 30 g/mg (AC-R-G), 32 g/mg (AC-R-H), 1.4 g/mg (AC-A-B) and 1.7 g/mg (AC-B- F) were observed. The low removal efficiency of the two last charcoals (from hemodialysis centers) can be explained by their low mesopore volumes, around 0.04 cm3/g in average, when compared to those of other charcoals taken as standards, AC-R-G and AC-R-H (0.20-0.39

In the dinamic adsorption dynamic of the [D-Leu1]MCYST-LR in a fixed bed of activated charcoal, the latter removed that toxin from a solution until the saturation of the bed, wherein the performance of this continuous adsorption process is affected, among other parameters, by the concentration of the entry solution (Gomes et al., 2001) and by operational conditionals such as particle size and fluid flow in the column (Inglezakis et al., 2002). According to Sag & Aktay (2001) and Barros et al. (2001), the solutions which are more concentrated saturate the bed faster, and small particles diminish the resistance to mass transfer. It is also known that an increase in the flow of the solution on the bed reduced its adsorption capacity, increasing the lenght of the mass transfer zone, because the phenomenon of mass transfer necessary for adsorption of [D-Leu1]MCYST-LR might not be able to continue in higher mass transfer rates, brought forth by an increase in the flow of the fluid (Watson, 1999). Besides, the bed flow can be deviated from the ideal because of the flow channeling due to insufficient material wettability. Those problems can reduce the adsorption process efficiency, because it is important that the column operates as close as possible to the flow/runoff conditions as the one observed for a tubular-type "plug flow" reactor. Hence, in order to correctely plan and operate the continuous adsorption process of that toxin in a AC fixed bed, as the one found in many water treatment stations of hemodialysis centers, it is necessary to study the kinetics and adsorption equilibrium for that toxin, besides knowing its adsorption dynamics in a fixed bed through the sizing of the breakthrough curves. The first step in a adsorption project for [D-Leu1]MCYST-LR in fixed bed column is the establishment of optimal conditions for preparing the process, that is, those that minimize the diffusional resistances both in the film and in the interior of adsorbent particles, thus favoring a greater interaction between the charcoals' active sites,

In this experiment, the previous studies of methylene blue adsorption in activated charcoal fixed bed have shown flows between approximately 8 and 12 mL/min and average particle size of 0.425 mm would be conditions that could reduce to minimum those mass transfer resistances without significant increase in charge loss on the bed, and thus be taken as a starting point for the planning of a continuous adsorption process for that toxin. Under these conditions, one should expect that the breakthrough curves come closer to a perfect degree, which is desirable (Mccabe et al., 2001). Thus, taking such conditions like particle size, flow and height of the bed, new experiments with solutions [D-Leu1]MCYST-LR were carried out aiming to evaluate the removal of this toxin from the treated water using fixed bed columns of AC-A-B charcoals and AC-B-F, besides the AC taken as standards AC-R-G

Aiming to recover the solution initially containing [D-Leu1]MCYST-LR kept in contact in a continuous fashion with an activated charcoal bed initially free from it, the concentration of this toxin in the exit of the bed was monitored, in function of the time, producing curves as

Standard international and national norms regarding the quality of activated charcoal for water treatment bring specifications primarily concerning the minimal iodine adsorption limit (iodine number), 600 mg/g (ABNT - EB-2133, 1991 and AWWA – B600-05) and 900 mg/g (ASTM - D 4607-96), making no mention to the minimal limits of methylene blue adsorption. It is known that iodine is a small molecule of approximately 0.8 nm and being thus associated with micropore adsorption. Therefore, the iodine number cannot be the sole specification of quality standard adopted for an activated charcoal destined for water treatment, because it is a well-applied parameter for microporous charcoals.

In Marroco, the activated charcoals destined for water treatment have in their specifications the minimal limits established for the methylene blue adsorption capacity of 180.0mg/g (Baçaoui et al., 2001). Thus, if we consider this minimal limit as a specification for the activated charcoal sampled in both the Hemodialysis Centers, we would see that none of these charcoals could be used for water treatment, because they are specifically microporous activated charcoals.

#### **3.2.1.1 Adsorption of [D-Leucine1]MCYST-LR**

From results obtained in the characterization of the hemodialysis centers AC regarding the pore volume and methylene blue adsorption, it was possible to choose the best AC in each center and use it in the adsorption studies with the cyanobacteria toxin. The adsorption kinetics of the [D-Leucine1]MCYST-LR on the charcoals was studied, from which we could estimate the efficiency of the removal of this cyanobateria toxin by those adsorbents. Other two activated charcoals produced by Albuquerque et al. (2005) (AC-R-G and AC-R-H, originating from the sugarcane bagasse and coconut tree endocarp) were also studied regarding their removal efficiencies for [D-Leu1]MCYST-LR on water.

Fig. 4. Adsorption kinetics of [D-Leucine1]MCYST-LR on the activated charcoals: (a) □ AC-B-F, ■ AC-A-B, and (b) ○ AC-R-G and ∆ AC- CS – F.

After 15 seconds of contact between the [D-Leu1]MCYST-LR and the charcoals, removal efficiencies of 7.40% (AC-R-G) and 26.47% (AC-R-H) were observed, compared to the very low results obtained by the AC-A-B and AC-B-F with removal efficiency of 1.96 and 2.26%, respectively. The adsorption kinetics evolved gradually with time until the adsorption balance was reached in around 30 minutes for the AC-R-G and AC-R-H, and of 60 minutes for the AC-A-B and AC-B-F (Figure 4). The removal efficiencies by these AC in the respective equilibrium times were of 62.31% (AC-R-G), 98.73% (AC-R-H), 4.3% (AC-B-F) and 3.7% (AC-A-B). Still in the equilibrium, toxin adsorption in the charcoals of 30 g/mg (AC-R-G), 32 g/mg (AC-R-H), 1.4 g/mg (AC-A-B) and 1.7 g/mg (AC-B- F) were observed. The low removal efficiency of the two last charcoals (from hemodialysis centers) can be explained by their low mesopore volumes, around 0.04 cm3/g in average, when compared to those of other charcoals taken as standards, AC-R-G and AC-R-H (0.20-0.39 cm3/g).
