Mobilization and loss of elements from roofing tiles

Abstract

Deposition, leaching and chemical transformation are processes that affect roofing tile and roof runoff water. Leaching experiments, with artificial rainwater in the laboratory, showed the presence of Na+, K+, Mg2+, Ca2+, Cl-, NO3-, SO42-, with a ratio of Ca2+ and SO42- suggesting gypsum dissolution. X-ray fluorescence (XRF) of the exposed roof tile showed depletion such as Mg, Al, Si, P, Ti and K at the surface of the tile and an enrichment of Fe and Mn which hinted at a process akin to laterite formation. However, calcium appeared to be enriched at the surface as gypsum (confirmed by X-ray diffraction) and to a lesser extent calcite, which is characteristic of deposits on building surfaces in cities

Mobilisation of trace elements on roofing tiles

“…Therefore, it is possible that roofing tiles sampled had been exposed to significant city activities such as motor vehicle emissions and domestic heating. 10 The exposures mean that the tile surfaces are covered with a dark layer; a mixture of atmospheric deposits, material leached from the tile along with biological growths.…”

“…Leaching experiments were performed by draining acid solutions across the tile outer surface to mimic runoff waters. 10 In these experiments the unglazed clay roof tile samples (25 cm  35 cm  1.5 cm) stood at an inclination of approximately of 45 . A series of solutions of pH 3, pH 4 and pH 5, respectively, were sprayed from a 250 ml bottle directly onto the tile surface.…”

“…These solutes arise from both the roofing material along with deposited and transformed air pollutants. 10 The diverse nature of roofing materials and weathering processes may affect the retention, deposition, different runoff behaviour and the nature of the surface (e.g. temperature, surface roughness and settlement of micro-organisms).…”

“…New roof surfaces represent a pollution sink until the highest load capacity is reached. 10,11 The roof surface can influence the deposition of pollutants because a coarse roof surface (tiled roofs) leads to more rapid deposition of atmospheric pollutants compared to the relatively smoother roof types (galvanised iron). Yaziz et al 12 found that coarser surfaces of roof allowed better entrapment of pollutants than a smoother roof.…”

“…Acidity and rainwater pH, although often seen as a key factor in leaching of solutes from roofs, proved not to be a key determinant in the loss of major elements from tiles. 10 Nevertheless, it is possible that acids play a role in releasing additional trace elements from roofs or metals within deposited particles. 21 Several studies have shown that tile roof runoff exceeded the limit concentrations for cadmium 22,23 and lead 12,22,23 based on World Health Organization (WHO) drinking water guidelines.…”