Towards A Better Understanding of Contaminant Fate in Plastic Plumbing Systems and Their Remediation

2019-01-17T14:26:53Z (GMT) by Xiangning Huang

This dissertation focused on better understanding the fundamental processes that control organic and inorganic contaminant interaction with plastic plumbing pipes. Plastic pipes are increasingly being installed for drinking water plumbing, but their role in affecting drinking water quality has received little study. It is well-known that plastic pipes can sorb and release organic contaminants and be difficult to decontaminate. Several problems were identified in the literature and through discussions with industry: (1) Past guidance issued to communities affected by petroleum contaminated water does not seem to specifically consider plastic plumbing pipe remediation, (2) investigators have also identified heavy metals can accumulate on pipe inner walls, (3) Others have proposed certain heavy metals can catalyze plastic water pipe degradation, (4) No nondestructive cleaning methods were found for removing metal scales from plastic pipes. These topics were a basis for studies conducted because lack of information inhibits greater protection of public health, safety, and welfare.

This dissertation involved the application of knowledge and techniques from the environmental engineering and science, polymer engineering, and material science disciplines. Chapter 1 focused on the response of copper and plastic pipes (i.e., chlorinated polyvinylchloride (cPVC), high-density polyethylene (HDPE), crosslinked polyethylene (PEX)) exposed to petroleum contaminated drinking water. Bench-scale results revealed that pipe rinsing followed by a single 3 day water stagnation period removed target monoaromatic hydrocarbons (MAH) from copper pipes, but much longer (>15 days) time was required for decontaminating cPVC, HDPC, and PEX pipes. Benzene, trimethylbenzene and polynuclear aromatic hydrocarbons, some of which are not typically considered in drinking water contamination investigations, were found desorbed into clean drinking water from pipes. Future plumbing decontamination guidance should consider the conditions necessary for plastic pipe remediation. Chapter 2 describes the influence of drinking water conditions on heavy metal contaminant – low density polyethylene (LDPE) pellet surface interactions. Mixed metal drinking water solutions were applied and contained Cu, Fe, Mn, Pb and Zn at 30 µg/L. LDPE was selected as the model polymer because of its prior use for piping in Europe, use in bench-scale studies by others, and similarity to products used for the manufacture of more complex materials in the USA (HDPE, PEX). As expected, metal loadings were about 5 times greater for aged LDPE pellets suspended in solution compared to new LDPE pellets. This difference was attributed to the aged plastic surfaces having oxygen containing functional groups, increased surface area, and enhanced hydrophilicity. Metal loading was lower at pH >9.5 and in the presence of dissolved organic contaminants. The presence of free chlorine and corrosion inhibitor also decreased metal adsorption onto LDPE pellets. These factors likely enabled metal precipitation thereby not allowing metal species to adsorb to LDPE pellets suspended in water. XPS results showed deposited metals (i.e., Cu, Pb, Zn) primarily consisted of hydroxides and oxides. To further understand heavy metal – plastic pipe interactions, Chapter 3 involved the use of metal and plastic pipe rigs and exhumed PEX plumbing pipes. Exhumed cold and hot water PEX pipes contained a noticeable amount of heavy metals (i.e., most abundant metals were 2049 mg/m2 Fe, 400 mg/m2 Ca, 438 mg/m2 Zn and 150 mg/m2 P). Metal release and deposition onto PEX pipe was examined using bench-scale pipe rigs that contained new PEX pipe, brass valves, and copper pipe. Two water matrices (pH 4 and 7.5) and two temperatures (23oC and 55oC) were explored. The pH 4 water often accelerated metal leaching from brass valves, and a greater amount of heavy metals deposited on PEX pipes at high water pH and temperature (pH 4 and 55oC) conditions. Oxygen containing functional groups were detected on PEX pipes connected to a brass valve or a brass valve combined copper pipe, but were not found on PEX pipe only (controls) samples, indicating that certain configurations may facilitate plastic pipe degradation. The last chapter describes the ability of a new lignin derived ligand to remove metal deposits from exhumed PEX plumbing pipes. When the ligand concentration was ≥ 5mM, more than 95% of sorbed metals (i.e., Cu, Fe, Mn, Pb and Zn) were removed. The ligand favored certain metals over others (Cu > Zn > Fe > Mn > Pb) and heavy metal removal mechanisms were proposed. This dissertation provides insights into the role of plastic pipes on drinking water quality. As plastic pipes continue to be installed, it is in the interest of public health, welfare, and safety to understand their role in positively and negatively affecting drinking water safety.