Heavy metal enrichment in drinking water pipe scales and speciation change with water parameters

Pipe scales that form in drinking water distribution systems (DWDS) can accumulate pollutants that may be re-released into bulk water, posing a significant threat to water safety. This study aims to evaluate the pollutant enrichment capacity of the pipe scale and identify speciation changes in heavy metals under variations in water quality. When the water quality conditions changed, the forms of inorganic metal elements in drinking water pipe scales also changed and the proportion of unstable forms increased, thereby increasing the risk of secondary pollution. Morphological analysis showed that the pipe scale samples had porous structures and large specific surface areas (the maximum was 52.94 m2/g, which is higher than that of many natural adsorbents), which could promote the accumulation of contaminants. XRD profiles also showed that the pipe scale samples were rich in substances with heavy metal adsorption capacities, such as Fe3O4. As the pH changed from 6 to 10, no significant difference in the release of heavy metals was found. The maximum release of Cu, Cr, As, Pb, and Cd at pH 8 was 0.56, 0.51, 1.82, 0.84, and 0.72 μg/g, respectively. Although the amounts were small, the speciation distribution of the heavy metals changed significantly. In addition, the proportion of unstable fractions increased, which increased the release risk of the pipe scale. The presence of humic acid accelerated the dissolution of organic matter and metals in the pipe scale, which further proved that the pipe scales were unstable and susceptible to water quality conditions. The pipe scales could not maintain stability when the water quality changed, and the DWDS should be regularly monitored and cleaned when necessary.