**2. Materials and methods**

**1. Introduction**

30 Agricultural Waste and Residues

may be present in wastewater.

leaching are negligible.

as water-insoluble Cr<sup>2</sup>

human activities.

soil profile.

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

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

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

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

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 consider-

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

2

−, CrO<sup>4</sup> 2

O7 2

O [11]; only a small part of it can be leached from soil. Chromium

−) and dichromate (Cr<sup>2</sup>

−, and Cr<sup>2</sup>

−), is generally mobile

O7 2 −. It

In solution, Cr (VI) can exist in three different ionic forms: HCrO<sup>4</sup>

O3¨H<sup>2</sup>

ing the adsorption and reduction capacity of soils [4, 13].

(VI), mainly present as chromate ions (CrO<sup>4</sup>
