Investigation of a Clay-Rich Aggregate for Use In Constructed Wetlands In the Eastern Yucatan

Akumal, a resort village on the eastern coast of Yucatan Peninsula, utilizes constructed wetlands for wastewater treatment. Because aggregate in these systems is largely composed of fine limestone gravel, it is vital to investigate other aggregate choices to increase filtration efficiency and improve vegetation growth.

Aggregate samples with appreciable clay content from a road side construction site have been studied as a potential replacement. This aggregate has a low average permeability of k = 0.00727 cm/s. Grain size distribution is poorly sorted and samples vary from unimodal to multimodal. The clay fraction of these aggregates consists of a mixture of clay-sized calcite, kaolinite and poorly crystalline aluminum oxide minerals. No environmentally problematic minerals such as sulfides or sulfates were identified.

Transmission electron microscopy (TEM) investigations indicates kaolinite particles are commonly 2.5 to 0.5 ìm in diameter and are commonly sub rounded to rounded. Selected area electron diffraction (SAED) patterns indicate the kaolinite crystals are highly crystalline with well defined pseudo hexagonal (hk0) nets. Oxyhydroxide particles are subhedral and tabular. These particles commonly vary in size with particles being approximately 25 nm x 100 nm to 100 x 300 nm in dimension. Selected area electron diffraction (SAED) patterns show orthorhombic nets. Energy dispersive spectroscopy (EDS) analysis of the oxyhydroxides particles indicates that they are Al-rich (93.39 to 73.61 wt% Al2O3) with lesser amounts of Si (25.16 to 4.66 wt% SiO2) and Fe (2.62 to 1.13 wt% Fe2O3). Some particles have minor amounts of P (0.23 to 1.51 wt% P2O5) while other have no P within detection limits (~0.1 wt %). Normalized plots of Al2O3-Fe2O3-SiO2 indicates that there is one chemical compositional trend.

This aggregate may be more beneficial media than present aggregates. This investigation identifies a local geologic resource that may be used to protect environments impacted by tourism.