6
Mobility of P, Fe and S at the sediment – water interface determined in experimental
columns using natural sediments and water from the Ciénega Tamasopo (SLP).
F. V. Pérez
1
, M.C. Alfaro
1
, R. Briones
2
, P.Medellín
3
1
Facultad de Ciencias Químicas, Universidad Autónoma de San Luis Potosí (UASLP), Av. Dr.
Manuel Nava 6, Zona Universitaria, 78210, San Luis Potosí, México. Tel (444) 8262300 ext 6565.
2
Instituto de Instituto de Metalurgia, UASLP, Sierra Leona 550. 78210, SLP, México.
3
Agenda
Ambiental, UASLP, Av. Dr. Manuel Nava, Zona Universitaria. 78210, SLP, México.
Keywords:
phosphorous, ratio (HS
-
/SO
4
2-
), sediment columns, porewater
Introduction
The natural wetland “Ciénega de Tamasopo” (RAMSAR Site 1814, 4MX095, 1364Ha) is located
in Tamasopo, Mex. Water inputs come from springs in the upper part of the basin, probably some
others discharging at the bottom of the basin, and rainwater. This wetland provides water for all
purposes to 15 small towns. It maintains an important biodiversity of flora and fauna including
some species identified with some level of risk. In the wetland, the recent sediments show en
enrichment with organic C, N and P; sulfide and sulfates are presents in the porewater and water
columns. Some authors have emphasized the role of sulfides and Fe in the availability and
mobility of sedimentary phosphorus (Roden and Edmonds, 1997). Giordani et al. (1996)
mediated laboratory experiments demonstrated that anoxia and bacterial sulfate reduction
stimulate ferric iron reduction, favoring phosphate release to porewater and thereby to the water
column. While, Azzoni et al. (2005) when evaluated the influence of iron and sulfide interactions
on sedimentary phosphorus cycling, observed that the great iron availability and reactivity acts as
efficient buffering capacity against sulfides.
Methodology
Six sediment cores were collected using a gravity sampler (Wildco 2404-A14) at the deepest site
of the Ciénega Tamasopo Wetland. In the laboratory, from each core, the most superficial wet
sediment was used to prepare experimental systems (acrylic cylinders 5.7cm diameter x 30 cm
height) creating a sediment and underlying water columns of 5-7cm and 20 cm thickness
respectively. Systems were constructed in duplicate using natural water from the wetland to
obtain 2 controls, 2 columns were the ratio P/Fe was studied and 2 more columns to study the P/S
ratio. Columns were monitored during 76 days (d) in the following stages: equilibration time
(9d), with oxygen limitation (18d), at oxidizing conditions (14d), with P addition (8d) and with
Fe or SO
4
2-
addition (16d). Changes were not applied to the control columns. In whole set of
columns, water samples were regularly taken at two depths with respect to the sediment-water
interface (1.5 cm above and 2.5 cm below) to study the mobilization of the elements between the
water and the sediments. Each time the following parameters were determined: pH, redox
potential, and the concentration of Fe
2+
, PO
4
3-
, SO
4
2-
, and Σ[HS
-
] using standard methods.
Preliminary, the effect of pH was tested by applying changes to the water column were the pH
was controlled initially to 8.5 and gradually decreased to 7.5 and finally to 6.5. Each change was
