**3.2. Physicochemical characterization of industrial wastewater**

**Table 3** shows the physicochemical characteristics of the wastewater used for irrigation of the soil columns. The concentration of total Chromium and Cr VI in the wastewater was high: 679.6 mg/L for total chromium and 559.5 mg/L for Cr VI. The concentration of copper was 18.5 mg/L; 0.64 mg/L for nitrates; 360.3 mg/L for sulfates; and 272.3 mg/L for chlorides. Electrical conductivity was 1576 μS/cm due to the presence of metals such as chromium, copper, chlorides, and sulfates. The pH of the water (3.4) is congruent with the presence of Chromium Species and 3D-Fluorescence Spectroscopy in a Soil Irrigated with Industrial… http://dx.doi.org/10.5772/intechopen.77181 33

(Cr VI) solution at standard concentrations of 5, 10, 15, 20, and 25 mg/L, in continuous stirring and constant temperature of 25°C. Five tests were performed for each depth of the soil column, with a contact time of half an hour. Subsequently, the tubes were centrifuged at 2500 rpm, and the

**Figure 1** shows the physico-chemical characteristics of the soil profile. The data obtained show a slight increase of pH as depth increases, with a value of 7.3 in the most superficial area and 7.5 at a depth of 50 cm. These pH values show that the soil is moderately alkaline, suggesting a medium availability of nutrients. The results of the electrical conductivity tests show a remarkable decrease along the soil column; the surface area has a value of 255.09 μS/cm and the deepest layer of 112.95 μS/cm. The lowest conductivity value (105.13 μS/cm) was observed at a depth of 30–40 cm (**Figure 1**). The cation exchange capacity remained constant throughout the soil column, with values of 30.20 Cmol/kg at a soil depth of 40–50 cm, and up to 38.21

The content of organic carbon (OC) gradually decreased as the depth of the soil column increased: from 9.96 g/kg in the surface layer to 2.29 g/kg a depth of 40–50 cm. The percentage of organic matter (OM) also decreased with increasing depth; the highest value (17.17%) was observed in the surface layer, and the lowest value (3.95%) in the deepest layer. The percentage of humidity, like the OC content, decreased along the soil column by up to 29%, from

Regarding the texture of the different layers of the soil column, we obtained the following results: the most superficial layer (0–10 cm) had sandy loam soil; at depths of 10–20 and 30–40 cm, the soil had a loamy texture, and in the intermediate layers of the column (20–

In general, all layers of the soil column had a loam texture. The literature on the subject states that a soil with medium alkaline pH is a sandy soil; this agrees with the texture data obtained in the present study, which showed a high sand content in all soil samples (**Table 2**). The results of this study also agree with the low amount of natural organic matter reported for

**Table 3** shows the physicochemical characteristics of the wastewater used for irrigation of the soil columns. The concentration of total Chromium and Cr VI in the wastewater was high: 679.6 mg/L for total chromium and 559.5 mg/L for Cr VI. The concentration of copper was 18.5 mg/L; 0.64 mg/L for nitrates; 360.3 mg/L for sulfates; and 272.3 mg/L for chlorides. Electrical conductivity was 1576 μS/cm due to the presence of metals such as chromium, copper, chlorides, and sulfates. The pH of the water (3.4) is congruent with the presence of

30 cm) and in the lowest layer 40–50 cm, the soil had a loamy-clay texture (**Table 1**).

these types of soils, as well as with deficiencies of B, Cu, Fe, Mn, Zn, and P [18].

**3.2. Physicochemical characterization of industrial wastewater**

supernatant was filtered, collected, and acidified for Uv–visible spectroscopy [14].

**3. Results and discussion**

32 Agricultural Waste and Residues

Cmol/kg at 10–20 cm depth.

**3.1. Physicochemical characteristics of the soil**

22.83% in the surface layer to 16.27% at a depth of 30–40 cm.

**Figure 1.** Physical and chemical characteristics of the soil sample as a function of depth. EC = electric conductivity, OC = organic carbon, CEC = cation exchange capacity.

chromium VI, since the species distribution diagram for chromium indicates that the chromium species present in waters with pH values ranging from 1 to 5 is chromium VI. The wastewater studied here came from an electroplating plant in the city of Toluca.

#### **3.3. Concentration of chromium VI in the soil solution throughout the soil column**

The concentration of chromium in samples of the soil-saturated solution collected from the soil profile at depth intervals of 10 cm (**Table 4**); it also shows the amount of chromium accumulated in each section of the soil column. The initial concentration of Cr VI in the water used


amount of accumulated chromium was found in the surface layer, which had the lowest percentage of clay. This demonstrates the participation of organic matter in the accumulation of

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

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35

Additional to the irrigation with wastewater, one irrigation was done with a compost solution (100 g of compost/1 L water) in order to observe the effect of dissolved organic matter on the accumulation of chromium in the soil column. The results showed that chromium was absent from the soil-saturated solution at any depth of the soil profile, indicating that the chromium was being immobilized or retained. After this irrigation, one more irrigation was carried out, maintaining the concentration of chromium in the water entering the soil profile at 559.5 mg/L (**Table 5**). In these irrigations, the concentration of Cr VI in the saturated water collected at the outlet of the soil column was lower than in the first irrigation, suggesting a higher accumula-

The theoretical amount of chromium that precipitated and accumulated in the soil during the first irrigation was 299 mg; when dissolved organic matter was added to the irrigation water, that amount increased to 326 mg. Using the solution containing dissolved organic matter improved the soil reduction conditions, causing the reduction of Cr (VI) to Cr (III), which produced a precipitate of chromium, either an oxide or hydroxide, that accumulated in the

Ce: concentration of Cr in the saturated solution at every 10 cm of depth (mg/L) A: amount of chromium retained (mg).

**Table 4.** Concentration of Cr VI in the soil-saturated solution (Ce). Retention capacity of Cr. Distribution coefficient of

**Peak A Peak B Peaks C and D Peak F** 0–10 158 (340/412) 313 (210/407) ND 121 (280/414) 10–20 255 (340/412) 540 (210/409) ND 177 (280/412)

**Depth (cm) Irrigation Kd T (°C) pH ORP (mV)**

0–10 260.0 299.0 11.5 23.0 5.7 102.1 10–20 240.0 20.0 0.8 22.0 5.7 103.3 20–30 80.0 160.0 2.0 22.0 5.8 96.5 30–40 40.0 40.0 40.0 22.0 5.9 92.4 40–50 40.0 0.0 0.0 21.0 5.9 91.2

**Ces A**

**Samples Intensity of fluorescence (Excitation/Emission (nm))**

20–30 33 (320/438) ND ND 30–40 67 (335/442) 200 (210/420) ND 40–50 93 (330/440) 206 (225/435) ND

**Table 5.** 3D-fluorescence characterization of the soil solution.

Kd, pH, and ORP along the soil column (q).

ND Undefined.

chromium (**Table 4**).

tion of chromium.

**Table 1.** Physicochemical characteristics of the soil column under study.


M sample.

**Table 2.** Textural characteristics of the soil column under study.


**Table 3.** Physicochemical characteristics of the wastewater used for irrigation.

for irrigation was 559.5 mg/L. The results show that the greatest accumulation of Cr occurred between 0 and 10 cm depth (299 mg/L), followed by the layer at 30–40 cm, with 160 mg/L Cr, a 50% decrease in the concentration of chromium VI present in soil solution.

The data on soil texture showed that the percentage of clay is low at 10–20 cm depths (10%), while at 20–30 cm is four times greater (41%). The percentage of organic matter is 17.17 and 8.45%. The CEC, however, is 35.22 and 35.29%, similar to the rest of the soil column. The highest amount of accumulated chromium was found in the surface layer, which had the lowest percentage of clay. This demonstrates the participation of organic matter in the accumulation of chromium (**Table 4**).

Additional to the irrigation with wastewater, one irrigation was done with a compost solution (100 g of compost/1 L water) in order to observe the effect of dissolved organic matter on the accumulation of chromium in the soil column. The results showed that chromium was absent from the soil-saturated solution at any depth of the soil profile, indicating that the chromium was being immobilized or retained. After this irrigation, one more irrigation was carried out, maintaining the concentration of chromium in the water entering the soil profile at 559.5 mg/L (**Table 5**). In these irrigations, the concentration of Cr VI in the saturated water collected at the outlet of the soil column was lower than in the first irrigation, suggesting a higher accumulation of chromium.

The theoretical amount of chromium that precipitated and accumulated in the soil during the first irrigation was 299 mg; when dissolved organic matter was added to the irrigation water, that amount increased to 326 mg. Using the solution containing dissolved organic matter improved the soil reduction conditions, causing the reduction of Cr (VI) to Cr (III), which produced a precipitate of chromium, either an oxide or hydroxide, that accumulated in the


Ce: concentration of Cr in the saturated solution at every 10 cm of depth (mg/L) A: amount of chromium retained (mg).

**Table 4.** Concentration of Cr VI in the soil-saturated solution (Ce). Retention capacity of Cr. Distribution coefficient of Kd, pH, and ORP along the soil column (q).


**Table 5.** 3D-fluorescence characterization of the soil solution.

for irrigation was 559.5 mg/L. The results show that the greatest accumulation of Cr occurred between 0 and 10 cm depth (299 mg/L), followed by the layer at 30–40 cm, with 160 mg/L Cr,

The data on soil texture showed that the percentage of clay is low at 10–20 cm depths (10%), while at 20–30 cm is four times greater (41%). The percentage of organic matter is 17.17 and 8.45%. The CEC, however, is 35.22 and 35.29%, similar to the rest of the soil column. The highest

a 50% decrease in the concentration of chromium VI present in soil solution.

**Table 3.** Physicochemical characteristics of the wastewater used for irrigation.

**Sample Depth (cm) Clay (%) Loam (%) Sand (%)** M1 0–10 10.0 30.0 60.0 M2 10–20 18.0 42.0 40.0 M3 20–30 41.0 46.0 13.0 M4 30–40 11.0 45.0 44.0 M5 40–50 34.0 54.0 12.0

**Characteristic Value** pH 3.4 Electrical conductivity (μS/cm) 1576 Cr VI (mg/L) 559.5 Cu (mg/L) 18.5 Nitrates (mg/L) 0.64 Sulfates (mg/L) 360.3 Chlorides (mg/L) 272.3 *Total Chromium (mg/L) 679.6*

**Soil Depth (cm) pH EC (μS/cm) CEC (Cmol/Kg) OC (g/Kg) OM % Humidity %** M1 0–10 7.30 255.09 35.22 9.96 17.17 22.83 M2 10–20 7.20 187.93 38.21 7.57 13.05 20.02 M3 20–30 7.40 124.74 35.29 4.90 8.45 16.63 M4 30–40 7.40 105.13 38.66 4.13 7.12 16.27 M5 40–50 7.50 112.91 30.20 2.29 3.95 19.08

**Table 2.** Textural characteristics of the soil column under study.

**Table 1.** Physicochemical characteristics of the soil column under study.

M sample.

M sample.

34 Agricultural Waste and Residues

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 water, the retention percentage reached 58%.

**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

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

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.

O3

. The amount of chromium in the solution decreased along the soil profile.

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

, and there was no presence

37

http://dx.doi.org/10.5772/intechopen.77181

values of each soil solution, the predominant Cr III species was Cr<sup>2</sup>

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

predominance of Cr<sup>2</sup>

O3
