Heavy Metal Content in Humic Acids by PIXE of Hyposodic Calcaric Phaeozems and Rendzic Leptosols Irrigated with Wastewater, DDR-063, Mexico.
Iván E. Reyes Solis1, Norma E. García Calderón1, Diana E. Servín Ruíz1, and Corina Solís2. (1) Facultad de Ciencias, UNAM, Circuito exterior s/n, Ciudad Universitaria, México, Mexico, (2) Instituto de Física, UNAM, Circuito exterior s/n, Ciudad Universitaria, México, Mexico
Wastewater reuse in agriculture is recognized to be favorable for crops productivity. However, it contributes heavy metals to agroecosystem. Wastewater irrigation has been carried out for 100 years in irrigation district 03 (DDR063) of Mezquital Valley, Hidalgo State, Mexico. Metal concentration in soils has been evaluated by several authors in Mezquital Valley, observing a direct relation between the increase of metal concentration and soil organic matter content. It is reported that Humic Substances (HS) adquiered complexes with heavy metals at several levels. With the aim to studythe effect of soil Humic Acids (HAs) to heavy metals fixation in soils with three different ages of irrigation we selected a wastewater-irrigated area in the DDR-03 at 99º14x to 99º18x W and 20º16x to 20º20x N at 2070 masl. Soil sampling was carried out on three plots of 625 m2 with irrigation periods corresponding to 5 (Plot 1), 50 (Plot 2), and 100 (Plot 3) years. Soil morphology was described in profiles made in the center of each plot. Soils were classified as Hyposodic Calcaric Phaeozem (Plot 1), Leptic Calcaric Phaeozem (Plot 2) and Rendzic Leptosol (Plot 3). Five soil samples were collected from 0-20 and 20-40 cm depth, air -dried and sieved. HAs were extracted by Dabin method. Soil samples and extracted HAs were air-dried, pulverized and pressed to pellets and total trace and heavy metals were determined using Particle Induced X-Ray (PIXE). Three replicates were prepared from each sample. Analysis by PIXE was performed with an external beam setup at the 3 MV9SDH NEC Pelletron accelerator in the Institute of Physics, UNAM, at 3 MeV proton beam for the primarily radiation. A Camberra LEGe detector was used to measure heavy metal content. Calibration of the detection system was carried out with pellets of HA Elliot soil reference material. The computer code GUPIX was used to obtain quantitative results. Cr, Co, Ni and Pb contents in bulk soils were higher than in HAs, only Cu were higher in HAs than in soils. Cu contents in HAs increase from 282 and 243 µg g-1 at Plot 1 to 465 and 467 µg g-1 at Plot 3, 0-20 and 20-40 cm depth respectively. Cr contents in HAs increase from 30,9 and 40,2 µg g-1 at Plot 1 to 63,5 and 51,9 µg g-1 at Plot 3, 0-20 and 20-40 cm depth respectively. Pb contents in HAs increase from 61,8 and 69,5 µg g-1 at Plot 1 to 83,4 and 76,8 µg g-1 at Plot 3, at 0-20 and 20-40 cm depth respectively. Cr, Cu and Pb contents showed a tendency to increase with irrigation time in HAs and soil. Lower level of Co were detected (8,6 µg g-1 ) at Plot 3 at 20-40 cm depth and higher level at 0-20 cm depth (32 µg g-1). In relation to Ni, lower level were detected (19,3 µg g-1) at Plot 2 at 20-40 cm depth and higher level (29,5 µg g-1) in Plot 3 at 20-40 cm. Co and Ni showed variable levels with irrigation time in both HAs and soils. Fe concentrations in HAs decrease with irrigation time from 40267,5 µg g-1 in Plot 1 at 0-20 cm to 25922,4 µg g-1 in Plot 3 at 0-20 cm, and were lower than in soil. Finally, the humic acid fraction had 20 to 90 percent of metal content in these soils. Cu exceed the llevels considered as potentially hazardous according to Mireles et al (2004). However, amount of HAs and alkaline pH keep most trace metals immobilized. Alkaline soil pHs contributes to complexation of metals to HAs. However, higher risk of metal availability increase due to pH decrease with irrigation time. It is possible that Cu accumulation in HAs fraction mainly through complexation to humic acids. For example, Cu may be complexed to one carboxylate and one neighbouring phenolate group. However, another binding mechanisms may be formation of clay–humus complexes. Key Words: Calcaric Phaeozems, Wastewater, Heavy Metals.