We used batch-type experiments to study Cr(VI) sorption/desorption on
granitic material, forest soil, pyritic material, mussel shell, and on
forest soil and granitic material amended with 12 t ha
Mining, industrial, and agricultural activities are the main sources of chromium pollution affecting the environment, notably the water and soil compartments (Alves et al., 1993; Di et al., 2006). Cr(III) is the chemically most stable form of chromium, whereas Cr(VI) is highly toxic and more easily mobilized. Mobilization of Cr(VI), and then risks of water pollution and even of transfer to the food chain, are strongly related to retention processes affecting the pollutant (Lilli et al., 2015).
Different bio-adsorbents have been tried to remove Cr(VI) from polluted environments, as was the case for some microorganisms and other natural sorbents (Schiever and Volesky, 1995). Schmuhl et al. (2001) found high Cr(VI) sorption on chitosan, with best results at pH 5. Blázquez et al. (2009) obtained Cr(VI) sorption > 80 % on olive waste at pH < 2, although sorption clearly diminished when pH increased. Good Cr(VI) sorption results were achieved using algae and cyanobacteria (Park et al., 2006; Gupta and Rastogi, 2008a, b), as well as using waste from the coffee and tea industries (Fiol et al., 2008; Duran et al., 2011).
Globally, it is necessary to increase the knowledge on Cr(VI) retention processes by sorbent materials. In this way, Fernández-Pazos et al. (2013) studied quantitative and kinetic aspects regarding Cr(VI) sorption/desorption on various solid media (fine and coarse mussel shell, unamended and mussel-shell-amended forest and vineyard soils, slate-processing fines and pyritic material). In addition to the kinetic characterization, it would be interesting to elucidate complementary aspects, such as the effects on Cr(VI) retention caused by changing pH, or the fractions where the retained Cr(VI) was bound, which can aid in estimating the degree of stability of that retention.
Therefore, the main objectives of this work are (a) to determine Cr(VI)
sorption/desorption when different Cr(VI) concentrations are added to a
granitic material, a forest soil, a pyritic material, and fine mussel shell,
as well as to the granitic material and the forest soil amended with 12 t ha
The materials used in this study are indicated in Table 1. The granitic
material (GM) was sampled in Santa Cristina (Ribadavia, Ourense Province,
Spain) and resulted from the evolution of a rocky substrate, similar to a C horizon,
nowadays exposed to the atmosphere after the elimination of the
upper horizons, then needing organic matter and nutrients to be restored, as
happens with granitic mine spoils. The forest soil (FS) was an A horizon,
with dominance of
FS, PM, and GM were sampled in a zigzag manner (20 cm depth), with 10 subsamples taken to perform each of the composite FS, PM, and GM final samples. These samples were transported to the laboratory to be air-dried and sieved through 2 mm. Finally, chemical determinations and trials were carried out on the < 2 mm fraction.
Materials investigated and abbreviations used to designate them.
The particle-size distribution of the materials was determined by using the
Robinson pipette procedure. A pH meter (model 2001, Crison, Spain) was used
to measure pH in water and in KCl (solid : liquid ratio 1 : 2.5). Total C and N
were quantified by means of the elemental Tru Spec CHNS auto-analyzer (LECO,
USA). Available P was determined according to Olsen and Sommers (1982). The
exchangeable cations were displaced using NH
Cr(VI) sorption and desorption as a function of the added concentration of the pollutant were studied as per Arnesen and Krogstrad (1998).
The adsorbents used were the materials previously mentioned (Table 1). Fernández-Pazos et al. (2013) found that the amendment of pyritic material with mussel shell had no positive effect on Cr(VI) retention, so this combination was discarded in the present study.
As in Fernández-Pazos et al. (2013), 3 g of each solid sample was added
with 30 mL NaNO
Immediately after finalizing each batch experiment corresponding to the
sorption trials, each sample was added with 30 mL of NaNO
To study sorption, triplicate samples (1 g each) of the sorbent materials
(Table 1) were added with 10 mL of solutions containing 5 mg L
Desorption was studied using triplicate samples (1 g each) of the same solid
materials as in the sorption trials, which were added with 10 mL of
solutions containing 100 mg L
Samples corresponding to the sorbent materials (Table 1) were added with a
NaNO
The statistical package SPSS 19.0 (IBM, USA) was used to check data for normality. Then, Pearson correlations were calculated.
Finally, Cr(VI) sorption data were fitted to the Freundlich model (Eq. 1). Fitting to the Langmuir model was not possible due to estimation errors being too high.
The formulation of the Freundlich equation is as follows:
General characteristics of the solid materials (average values for three replicates, with coefficients of variation always < 5 %).
Element
Table 2 shows data of some basic characteristics of the solid materials used in this study.
Figure 1 shows that Cr(VI) sorption increased with Cr(VI) concentration in
the equilibrium solution, which was directly related to the increase in the
Cr(VI) concentration added. The same behavior was observed by
Fernández-Pazos et al. (2013) using mussel shell, pyritic material,
forest soil and slate-processing fines; by Rawajfih and Nsour (2008) using
Sorption points corresponding to the pyritic material
Fitting of the various materials to the Freundlich model.
Desorbed Cr (mg kg
GM: granitic material; FS: forest soil; PM: pyritic material; Sh: mussel shell.
Significant correlations (
Sorption data were satisfactory fitted to the Freundlich model through nonlinear regression (Table 3), as other authors found for various bio-sorbents (Cetinkaya-Donmez et al., 1999; Prakasham et al., 1999). Due to the fact that the Freunlich model considers that, theoretically, sorption could be infinite, the fitting to this equation means that sorption maximum would not be easily predictable for these materials.
Significant correlations (
Table 4 shows that the lowest Cr(VI) desorption corresponded to the pyritic
material (0.4–0.8 %), whereas mussel shell by itself released between 17
and 26 % of the amounts previously adsorbed. When 12 t ha
Relation between pH and sorbed Cr (mg kg
With the exception of panel a, Fig. 2 shows an overall increase in Cr(VI)
sorption as a function of decreasing pH values in the equilibrium solutions.
Similarly, different authors have indicated that optimum pH values for Cr(VI)
sorption are between 1 and 2.5 (Huang and Wu, 1977; Boddu et al., 2003;
Mohanty et al., 2006; Rawajfih and Nsour, 2008; Vinodhini and Nilanjana, 2009;
Wang et al., 2009), due to a higher density of positive charges on the
adsorbent surface, thus facilitating the binding to chromium anions that
dominate at these very acid pH values (HCrO
Figure 3 shows chromium desorption percentage for the various materials
after being added with 100 mg L
Relation between desorbed Cr(VI) (%) and pH for pyritic
material
Percentages of the various fractions of chromium sorbed after the
addition of 100 mg L
Figure 4 shows the results corresponding to the fractionation of the
adsorbed Cr(VI), after 24 h (Fig. 4a), 1 week (Fig. 4b), and 1 month of
incubation (Fig. 4c). The soluble fraction (the most labile, Gleyzes et al., 2002;
constituted by exchangeable and carbonate-bound forms) was 95 %
of the adsorbed Cr in mussel shell, and 80 % in granitic material, after
24 h of incubation. The mussel shell amendment caused the soluble
fraction in the granitic material to increase to 95 %, with parallel
diminution of other more stable fractions, probably due to Cr binding to
carbonates present in the shell. Mussel shell and the granitic material
(unamended or amended) did not show relevant modifications in the
percentage of the soluble fraction for more extended periods of incubation
(1 week and 1 month). At 24 h of incubation, the soluble fraction was 35 %
for forest soil, and 7 % for the pyritic material. The value did not
suffer relevant changes with time for the latter, but in the case of forest
soil it decreased to 17 and to 11 % when incubation time was 1 week
and 1 month, respectively, due to the increase of a more stable fraction
(the oxidizable one, related to organic matter). The mussel shell
amendment did not cause remarkable changes in the content of the soluble
fraction of forest soil. At 24 h of incubation, the reducible fraction (Cr
bound to Fe and Al oxides and oxy-hydroxides) represented less than 12 %
in mussel shell, as well as in amended and unamended forest soil and
granitic material, but more than 35 % in the pyritic material, which can
be due to its Fe
The pyritic material showed the highest Cr(VI) retention capacity among the
solid substrates studied, while the lowest corresponded to the granitic
material. The forest soil presented high sorption potential when pH was acid
and the Cr(VI) concentration added was < 10 mg L
This study was funded by the Ministerio de Economía y Competitividad (Government of Spain), grant numbers CGL2012-36805-C02-01-02. Edited by: A. Jordán