**3.5. Chromium species in wastewater and soil solution along the soil profile**

The species distribution diagram of Cr VI and Cr III were built based on the ORP and pH data, using the Hydra and Medusa programs [19]. **Figure 2** shows the Eh-pH diagram of chromium in the wastewater; it shows that the HCrO<sup>4</sup> − ion is the Cr VI species that predominated in both oxidized and reduced environments at pH values of 0–6.6. The CrO<sup>4</sup> 2 − ion predominated at values of pH > 6.0. According to the diagram, the Cr III species that predominated in the solution, depending on the pH of the water, was Cr3+. Thus, Cr VI species such as HCrO<sup>4</sup> −, and Cr III species such as Cr3+, entered the soil with the irrigation water, the latter in smaller quantities.

**Figure 2.** Stability diagram of chromium species (Eh-pH) in wastewater with high chromium content. Cr VI concentration: 10.78 mM; Cr III concentration: 2.31 mM; pH 3.4. (a) Cr VI species and (b) Cr III species.

**Figure 3** shows the stability diagrams of chromium (Eh-pH) along the soil column during irrigation with wastewater with high chromium content. As observed in **Figure 4**, and based on the pH values of each soil solution, the predominant Cr III species was Cr<sup>2</sup> O3 , and there was no presence of Cr VI species, confirming that chromium III species precipitate and accumulate in soil as chromium oxide combined with natural organic matter [18, 19]. The redox potential measured in the soil solution during the irrigation was between 91.2 and 103.3 mV, with pH values between 5.67 and 5.90. In the species distribution diagram of chromium, these intervals correspond to the area of predominance of Cr<sup>2</sup> O3 . The amount of chromium in the solution decreased along the soil profile.

#### **3.6. 3D-fluorescence of dissolved organic matter in the soil solution**

soil matrix [18, 19]. Of the 559.5 mg/L of chromium that were added to the soil column with the first irrigation, 53% was retained; after adding dissolved organic matter to the irrigation

It was observed that the concentration of chromium (VI) in the wastewater decreased along the soil column. **Table 4** shows the concentration of chromium (VI), the oxidation–reduction potential and pH along the soil column. The samples show a direct effect of dissolved organic matter on the soil conditions that facilitate the accumulation of chromium. The irrigation results show that dissolved organic matter improved the soil reduction conditions, which promoted the accumulation by precipitation of chromium species. The soil reduction conditions are represented by the ORP values in the soil-saturated solution at the outlet of the soil column

The initial ORP value of the irrigation water was 274.9 mV; it decreased by up to 66% with each additional 10 cm of depth. The pH of the irrigation water changed from 3.4 to 5.6. It should be mentioned that the wastewater used for irrigation stayed in the soil column for

The species distribution diagram of Cr VI and Cr III were built based on the ORP and pH data, using the Hydra and Medusa programs [19]. **Figure 2** shows the Eh-pH diagram of chromium

values of pH > 6.0. According to the diagram, the Cr III species that predominated in the solu-

III species such as Cr3+, entered the soil with the irrigation water, the latter in smaller quantities.

**Figure 2.** Stability diagram of chromium species (Eh-pH) in wastewater with high chromium content. Cr VI concentration:

10.78 mM; Cr III concentration: 2.31 mM; pH 3.4. (a) Cr VI species and (b) Cr III species.

tion, depending on the pH of the water, was Cr3+. Thus, Cr VI species such as HCrO<sup>4</sup>

− ion is the Cr VI species that predominated in both

2

− ion predominated at

−, and Cr

**3.5. Chromium species in wastewater and soil solution along the soil profile**

oxidized and reduced environments at pH values of 0–6.6. The CrO<sup>4</sup>

water, the retention percentage reached 58%.

15 min, which shows that the soil had good drainage.

in the wastewater; it shows that the HCrO<sup>4</sup>

**3.4. Irrigation with wastewater**

36 Agricultural Waste and Residues

during direct irrigation.

The 3D fluorescence spectra of the soil-saturated solution, based on the fluorescence data obtained (**Table 5**), show two peaks: A and B (**Figure 4**). These peaks are located within a

**Figure 3.** Stability diagram of chromium (Eh-pH) in the soil solution: (a) depth (0–10 cm), ionic strength 0.005 M, [Cr VI] 5.01 mM, [Cr III] 1.07 mM; (b) (10–20 cm), ionic strength 0.004 M [Cr VI] 4.62 mM and [Cr III] 0.99 mM; (c) (20–30 cm), ionic strength 0.001 M [Cr VI] 0.77 mM and [Cr III] 0.17 MM; (d) (30–50 cm), ionic strength 0.001 M [Cr VI] 0.77 mM and [Cr III] 0.17 mM for the entire pH range and for the range of pH measurements and redox potential of the samples.

range of excitation/emission wavelengths of 320–340 ex/412–440em and 210–225ex/407– 435em, with intensities of 33–255 and 200–540, respectively. The A and B peaks are within the regions corresponding to humic and fulvic acids [16]. The spectra did not show peaks of type C and D, which are associated with the presence of organic material of anthropogenic origin. However, a peak (F) between the peaks A and B could be considered a humic acid, given the region in which it is located, although it could also be associated with some organic synthetic

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The highest fluorescence intensity was recorded at a depth of 0–20 cm, in agreement with the fact that the surface layer had the greater presence of natural organic matter (**Figure 4**). As mentioned before, the chromium accumulated in the soil profile may combine with two types of natural organic matter, humic and fulvic acids. The fluorescence spectra showed the evolution of humic substances [15, 17]. In the first 20 cm of the soil profile, the fluorescence intensity of humic acids is high (158 at 10 cm and 255 at 20 cm), but decreases with increasing depth by up to 87%. The fluorescence intensity of fulvic acids is higher than that of humic acids (313

Because fulvic acids are more mobile than humic acids, it is logical to find them throughout the soil profile (**Figure 4**). These acids can predominate in natural waters and have a high degree of anionic charge, which favors the formation of stable complexes with cations such as chromium [15, 20, 21]. It is worth noting the amount of chromium accumulated in the soil profile (**Table 6**); the greatest accumulation of chromium was observed in the first 10 cm of the column, where there was a greater presence of organic matter and where peak F was observed. At a depth 30–40 cm, the accumulated chromium was only 63% of the level found in the surface layer; however, at this depth, the organic matter content was 60% lower than on the surface layer, while the percentage of clay was the

The fluorescence spectra indicated that most of the dissolved organic matter present in the surface layer and at a depth of 30–40 cm were fulvic acids. As mentioned earlier, fulvic acids are more mobile and tend to form complexes with Cr (III) cations, which suggests that, given the low amount of Cr (III) present in the wastewater used for irrigation, it precipitated mostly in the first 20 cm of the soil profile, which explains its absence from the soil-saturated solution. The Cr (VI) species present in the solution collected at the outlet of the soil column may not

**Table 6** shows the chromium retention results of the soil, based on the retention capacity (q) and equilibrium concentrations (Ce) determined by Bach tests at different concentrations of chromium. The table shows that the retention capacity of the soil increased as the equilibrium concentration of chromium increased. However, a constant value was not reached, which is usually observed when the soil reaches a saturation point; this can be explained by the low concentration of chromium in the standard solutions. The results of the Bach tests showed that the dispersion coefficient of chromium in the soil was higher (Kd 8.36) in the surface layer

have passed enough time in the soil column to convert to Cr (III).

**3.7. Chromium retention capacity of the soil**

material or a complex of Cr and humic acids.

and 540) and decreases less with depth (62%).

same in both layers.

**Figure 4.** 3D fluorescence spectra of the soil-saturated solution along the soil profile.

range of excitation/emission wavelengths of 320–340 ex/412–440em and 210–225ex/407– 435em, with intensities of 33–255 and 200–540, respectively. The A and B peaks are within the regions corresponding to humic and fulvic acids [16]. The spectra did not show peaks of type C and D, which are associated with the presence of organic material of anthropogenic origin. However, a peak (F) between the peaks A and B could be considered a humic acid, given the region in which it is located, although it could also be associated with some organic synthetic material or a complex of Cr and humic acids.

The highest fluorescence intensity was recorded at a depth of 0–20 cm, in agreement with the fact that the surface layer had the greater presence of natural organic matter (**Figure 4**). As mentioned before, the chromium accumulated in the soil profile may combine with two types of natural organic matter, humic and fulvic acids. The fluorescence spectra showed the evolution of humic substances [15, 17]. In the first 20 cm of the soil profile, the fluorescence intensity of humic acids is high (158 at 10 cm and 255 at 20 cm), but decreases with increasing depth by up to 87%. The fluorescence intensity of fulvic acids is higher than that of humic acids (313 and 540) and decreases less with depth (62%).

Because fulvic acids are more mobile than humic acids, it is logical to find them throughout the soil profile (**Figure 4**). These acids can predominate in natural waters and have a high degree of anionic charge, which favors the formation of stable complexes with cations such as chromium [15, 20, 21]. It is worth noting the amount of chromium accumulated in the soil profile (**Table 6**); the greatest accumulation of chromium was observed in the first 10 cm of the column, where there was a greater presence of organic matter and where peak F was observed. At a depth 30–40 cm, the accumulated chromium was only 63% of the level found in the surface layer; however, at this depth, the organic matter content was 60% lower than on the surface layer, while the percentage of clay was the same in both layers.

The fluorescence spectra indicated that most of the dissolved organic matter present in the surface layer and at a depth of 30–40 cm were fulvic acids. As mentioned earlier, fulvic acids are more mobile and tend to form complexes with Cr (III) cations, which suggests that, given the low amount of Cr (III) present in the wastewater used for irrigation, it precipitated mostly in the first 20 cm of the soil profile, which explains its absence from the soil-saturated solution. The Cr (VI) species present in the solution collected at the outlet of the soil column may not have passed enough time in the soil column to convert to Cr (III).

#### **3.7. Chromium retention capacity of the soil**

**Figure 4.** 3D fluorescence spectra of the soil-saturated solution along the soil profile.

38 Agricultural Waste and Residues

**Table 6** shows the chromium retention results of the soil, based on the retention capacity (q) and equilibrium concentrations (Ce) determined by Bach tests at different concentrations of chromium. The table shows that the retention capacity of the soil increased as the equilibrium concentration of chromium increased. However, a constant value was not reached, which is usually observed when the soil reaches a saturation point; this can be explained by the low concentration of chromium in the standard solutions. The results of the Bach tests showed that the dispersion coefficient of chromium in the soil was higher (Kd 8.36) in the surface layer


**Table 6.** Results of Ce, q and Kd along the soil column.

of the soil column (0–10 cm depth), which had the largest concentration of organic matter and the lowest concentration of chromium (5 mg/L). This behavior shows the affinity of chromium for organic matter. At a depth of 20–30 cm, where the percentage of clay was the largest (41%) and the dispersion coefficient of chromium was Kd 0.85.

Starting at a depth of 20 cm, the dispersion coefficient increased while the concentration of chromium in the solution increased, except for interval of 20–30 cm depth, which showed an opposite behavior. Bach tests have been designed to study adsorption equilibria in a continuously stirred soil suspension. These tests are based on a physical model of a completely dispersed soil particle system where the entire surface of the soil is exposed and available to interact with chromium. These tests do not represent real natural conditions, since they assume a closed system in which soil particles have the highest adsorption capacity, and a practically null flow rate [21–23]. However, they are a good tool to try to represent and understand the behavior of contaminants. The Bach tests performed in this study were carried out with a contact time of 30 min, twice the residence time of the water in the soil column, in order to represent real conditions.

A linear isotherm was used to describe the adsorption processes that took place in the soil; this allowed us to describe the distribution of chromium between the soil and the solution [22, 23]. The isotherms generated by the Bach tests (**Figure 5**) show the retention capacity of chromium in the soil (q) versus the equilibrium concentration of chromium (Ce).

conditions, the highest accumulation of chromium occurred in the first 10 cm of the soil column, in agreement with the results of the Bach tests, which showed that chromium has a high affinity for organic matter, as evidenced by the Kd value (**Table 6**). As mentioned before, the dispersion coefficients determined by the Bach tests decreases with depth in the soil column from 8.38 to 2.13. The fluorescence intensity of humic acids also decreased with depth; the highest fluorescence intensity of these humic substances was recorded in the first 10 cm of the

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**Figure 5.** Chromium adsorption isotherms along the soil column. Contact time: 30 min. Temperature: 25°C.

The dispersion coefficient of humic acids was similar to that of chromium, so it is possible to assume that Cr (VI) species complexed with humic acids or were reduced to Cr (III) in the

soil column, where the dispersion coefficient was higher.

At low concentrations of chromium, the adsorption isotherm for the soil is linear. **Figure 5** shows that with a low concentration of chromium and a contact time range of 0–25 min, 45% of chromium is removed from the solution in the first 10 cm of the soil column, 20% at 10–20 cm, 7.8% at 20–30 cm, 13% at 30–40 cm, and 17% at 40–50 cm.

The removal percentage of chromium from the solution decreased as the concentration of chromium in the solution increased. The removal percentage of chromium also decreased throughout the soil column, with the highest percentage at 20–30 cm depth. Under nonequilibrium Chromium Species and 3D-Fluorescence Spectroscopy in a Soil Irrigated with Industrial… http://dx.doi.org/10.5772/intechopen.77181 41

of the soil column (0–10 cm depth), which had the largest concentration of organic matter and the lowest concentration of chromium (5 mg/L). This behavior shows the affinity of chromium for organic matter. At a depth of 20–30 cm, where the percentage of clay was the largest (41%)

Starting at a depth of 20 cm, the dispersion coefficient increased while the concentration of chromium in the solution increased, except for interval of 20–30 cm depth, which showed an opposite behavior. Bach tests have been designed to study adsorption equilibria in a continuously stirred soil suspension. These tests are based on a physical model of a completely dispersed soil particle system where the entire surface of the soil is exposed and available to interact with chromium. These tests do not represent real natural conditions, since they assume a closed system in which soil particles have the highest adsorption capacity, and a practically null flow rate [21–23]. However, they are a good tool to try to represent and understand the behavior of contaminants. The Bach tests performed in this study were carried out with a contact time of 30 min, twice the residence time of the water in the soil column, in order

A linear isotherm was used to describe the adsorption processes that took place in the soil; this allowed us to describe the distribution of chromium between the soil and the solution [22, 23]. The isotherms generated by the Bach tests (**Figure 5**) show the retention capacity of

At low concentrations of chromium, the adsorption isotherm for the soil is linear. **Figure 5** shows that with a low concentration of chromium and a contact time range of 0–25 min, 45% of chromium is removed from the solution in the first 10 cm of the soil column, 20% at

The removal percentage of chromium from the solution decreased as the concentration of chromium in the solution increased. The removal percentage of chromium also decreased throughout the soil column, with the highest percentage at 20–30 cm depth. Under nonequilibrium

chromium in the soil (q) versus the equilibrium concentration of chromium (Ce).

10–20 cm, 7.8% at 20–30 cm, 13% at 30–40 cm, and 17% at 40–50 cm.

and the dispersion coefficient of chromium was Kd 0.85.

**Table 6.** Results of Ce, q and Kd along the soil column.

40 Agricultural Waste and Residues

to represent real conditions.

**Figure 5.** Chromium adsorption isotherms along the soil column. Contact time: 30 min. Temperature: 25°C.

conditions, the highest accumulation of chromium occurred in the first 10 cm of the soil column, in agreement with the results of the Bach tests, which showed that chromium has a high affinity for organic matter, as evidenced by the Kd value (**Table 6**). As mentioned before, the dispersion coefficients determined by the Bach tests decreases with depth in the soil column from 8.38 to 2.13. The fluorescence intensity of humic acids also decreased with depth; the highest fluorescence intensity of these humic substances was recorded in the first 10 cm of the soil column, where the dispersion coefficient was higher.

The dispersion coefficient of humic acids was similar to that of chromium, so it is possible to assume that Cr (VI) species complexed with humic acids or were reduced to Cr (III) in the presence of these humic substances. Moreover, the R<sup>2</sup> values found along the soil column showed that the isotherm can describe the sorption or complexation behavior of chromium with humic acids.

**Author details**

México, México

**References**

Jose Alfredo Ramos Leal<sup>4</sup>

México, México D.F., México

Tecnológica, San Luis Potosí, México

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Rosa María Fuentes-Rivas1,4\*, Francisco Martin-Romero<sup>2</sup>

\*Address all correspondence to: rmfuentesr@uaemex.mx

1 Facultad de Geografía, Universidad Autónoma del Estado de México, Toluca, Estado de

2 Departamento de Geoquímica, Instituto de Geología, Universidad Nacional Autónoma de

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3 Centro Interamericano de Recursos del Agua, Toluca, Estado de México, México

4 División de Geociencias Aplicadas, Instituto Potosino de Investigación Científica y

Reyna Maria Guadalupe Fonseca-Montes de Oca3

, Daury García Pulido<sup>3</sup>

, Janete Moran Ramírez<sup>4</sup>

Chromium Species and 3D-Fluorescence Spectroscopy in a Soil Irrigated with Industrial…

,

43

and

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An X-ray diffraction analysis was performed to verify the precipitation of chromium in the soil. The results showed the presence of the species Cr<sup>2</sup> O5 , Cr5 O12, CrO<sup>2</sup> , and Cr7 C3 in the soil layer with the greatest amount of organic matter, confirming the removal of chromium by precipitation. Considering that most of the reducers used here and reported in the literature are less effective at alkaline pH values [5], natural organic matter (humic acid) could be used for the remediation of soils and waters contaminated with Cr (VI).
