**1. Introduction**

Wastewater reuse in farming Mexican represents a valuable resource in agricultural production due to the irrigation supply and considerable nutrients input to the soil. Negative environmental effects may result from long-term wastewater application due to heavy metal accumulation in soils, increasing amounts of highly mobile, and easily mobilizable metal fractions, as well as crops uptake [1, 2]. Among the solid reactive components present in the soil, organic matter (OM), which has a high sorption capacity for metal ions [2–5], plays a very important role in soil fertility. The positive effects of organic matter are due to the fact that it benefits the aggregation of soil particles, improving aeration, permeability, resistance to erosion, and water retention. Regarding the chemical function of organic matter, it is based on its high cation retention capacity, which contributes greatly to the control of soil acidity, nutrient recycling and the detoxification of dangerous compounds such as heavy metals that are incorporated into soils by industrial wastewater [6]. Chromium is among the metals that may be present in wastewater.

**2. Materials and methods**

**2.1. Soil sampling and irrigation**

teristics of the soil: pH, Ce, CEC, MOS, moisture, and texture.

**2.2. Sampling and characterization of wastewater**

diphenylcarbazide spectrophotometry) [14].

**2.4. Chromium retention capacity of soil**

**2.3. Fluorescence spectroscopy and X-ray diffraction**

fluorescence spectrum were determined in the collected solution samples.

dard methods [14], were pH, electrical conductivity (EC), nitrates (NO<sup>3</sup>

Three soil samples were taken in an agricultural area, previously conditioned with the addition of organic matter in order to ensure a high content of organic matter in the soil. Sampling was performed vertically by inserting a PVC tube 10 cm in diameter at a depth of 50 cm at a random point in the area, so that three complete soil columns were obtained. The sampling and transfer was carried out hermetically to guarantee the inviolability of the sample until the arrival at the laboratory. The first profile was used to determine the physicochemical charac-

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

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

31

The other two profiles were irrigated with wastewater from an electroplating industry (559.5 mg/L Cr VI and 20.1 mg/L de Cr III); the irrigation was carried out on a single occasion in order to saturate the soil with Cr. About 10-cm deep holes were successively drilled in the soil profiles until reaching a depth of 50 cm. A sample of the saturated solution was obtained from each of these holes to observe the decrease of the concentration of chromium in the saturated solution after crossing 10 cm of soil. The ORP, Cr VI concentration [14], and 3D

The water sampling was carried out in the discharge of wastewater from an electroplating industry, located in the City of Toluca, State of Mexico. A sample composed of 5 L of residual water was collected, which was integrated with five individual samples of 1 L each, taken from the wastewater discharge every 15 min. The parameters, determined according to stan-

chlorides (Cl−), total chromium (by atomic absorption spectrometry) and chromium VI (by

The 3D fluorescence analysis was performed [15–17]. A Perkin Elmer fluorescence spectrophotometer LS55 was used, with 150 watts xenon lamp as excitation source. In the characterization of the samples, 45 individual emission spectra were obtained at emission wavelengths (λem) between 250 and 600 nm with intervals of 5 nm and, collected at excitation wavelengths (λexc) between 200 and 450 nm. The samples were analyzed at a concentration < 2 mg/L COD [15, 17]. A 290 nm filter was used in all analyses to eliminate Raleigh peak light interference. The excitation-emission matrix (EEM) of distilled water was subtracted from the EEM of the industrial wastewater samples to eliminate interference caused by Raman peaks. In order to verify the presence of chromium retained in the soil, the X-ray diffraction analysis was performed.

Batch tests were carried out to determine the Cr accumulation capacity of soil; 1 g of dry soil, sieved to a particle size of 0.002 mm, was placed in a glass tube together with 10 ml of a chromium

−), sulfates (SO<sup>4</sup>

2 ),

Chromium is a trace component in the Earth's crust (0.02%) that is essential for animal and human life, but not for plants. It is a natural element present in water, sediments, rocks, soils, plants, biota, animals, and volcanic emissions. The main oxidation forms of chromium are trivalent chromium and hexavalent chromium, each with opposite properties [7]. The total concentration of chromium in the lithosphere is between 69 and 100 mg/kg [7, 8]. The two forms of chromium have different effects on living organisms: Chromium (III) is apparently useful and harmless at reasonable concentrations, while Chromium (VI) is extremely toxic. Moreover, Chromium (III) is not mobile in soil; therefore, the risks of leaching are negligible.

In solution, Cr (VI) can exist in three different ionic forms: HCrO<sup>4</sup> −, CrO<sup>4</sup> 2 −, and Cr<sup>2</sup> O7 2 −. It can also exist in the form of complex anions that are soluble in water and may persist in it. In surface water rich in organic content, Cr (VI) has a much shorter shelf life [9]. The presence of each ionic form of chromium in solution depends on the pH [10]. Chromium is present in soils as water-insoluble Cr<sup>2</sup> O3¨H<sup>2</sup> O [11]; only a small part of it can be leached from soil. Chromium (VI), mainly present as chromate ions (CrO<sup>4</sup> 2 −) and dichromate (Cr<sup>2</sup> O7 2 −), is generally mobile and is sometimes part of crystalline minerals [7, 12]. In soil, Cr (VI) tends to be reduced to Cr (III) by organic matter. The chromium present in the environment is mainly derived from human activities.

Chromium (III) converts to Chromium (VI) only in some soils, particularly those that are rich in manganese oxides, poor in organic matter and with high oxidation–reduction potential. In contrast, the conversion of Chromium (VI) to Chromium (III) is very common and easy, and is thus very difficult to find hexavalent chromium forms in the soil solution or in leaching waters [7, 13]. The mobility of chromium in the lithosphere can only be evaluated by considering the adsorption and reduction capacity of soils [4, 13].

Accordingly, the aim of this study was to identify the chromium species present in soil and the saturated solution during irrigation with wastewater and characterize the dissolved organic matter, through the 3D fluorescence spectroscopy analysis, and its evolution in the soil profile.
