Impact of sodium silicate on lead release and colloid size distributions in drinking water

Inhibition of Hexavalent Chromium Release from Drinking Water Distribution Systems: Effects of Water Chemistry-Based Corrosion Control Strategies

In drinking water distribution systems, the oxidation of zerovalent chromium, Cr(0), in iron corrosion scales by chlorine residual disinfectant is the dominant reaction to form carcinogenic hexavalent chromium, Cr(VI). This study investigates inhibitive corrosion control strategies through adjustments of chemical water parameters (i.e., pH, silicate, phosphate, calcium, and alkalinity) on Cr(VI) formation through oxidation of Cr(0)(s) by free chlorine under drinking water conditions. The results show that an increase in pH, silicate, alkalinity, and calcium suppressed Cr(VI) formation that was mainly attributed to in situ surface precipitation of new Cr(III) solids on the surface of Cr(0)(s), including Cr(OH)3(s), Cr2(SiO3)3(s), CrPO4(s), Cr2(CO3)3(s), and Cr10Ca(CO3)16(s). The Cr(III) surface precipitates were much less reactive with chlorine than Cr(0)(s) and suppressed the Cr redox reactivity. The concentration of surface Cr(III) solids was inversely correlated with the rate constant of Cr(VI) formation. Adding phosphate either promoted or inhibited the Cr(VI) formation, depending on the phosphate concentration. This study provides fundamental insight into the Cr(VI) formation mechanisms via Cr(0) oxidation by chlorine and the importance of surface precipitation of Cr(III) solids with different corrosion control strategies and suggests that increasing the pH/alkalinity and addition of phosphate or silicate can be effective control strategies to minimize Cr(VI) formation.

Impact of corrosion inhibitors on antibiotic resistance, metal resistance, and microbial communities in drinking water

Corrosion inhibitors, including zinc orthophosphate, sodium orthophosphate, and sodium silicate, are commonly used to prevent the corrosion of drinking water infrastructure. Metals such as zinc are known stressors for antibiotic resistance selection, and phosphates can increase microbial growth in drinking water distribution systems (DWDS). Yet, the influence of corrosion inhibitor type on antimicrobial resistance in DWDS is unknown. Here, we show that sodium silicates can decrease antibiotic resistant bacteria (ARB) and antibiotic-resistance genes (ARGs), while zinc orthophosphate increases ARB and ARGs in source water microbial communities. Based on controlled bench-scale studies, zinc orthophosphate addition significantly increased the abundance of ARB resistant to ciprofloxacin, sulfonamides, trimethoprim, and vancomycin, as well as the genes sul1, qacEΔ1, an indication of resistance to quaternary ammonium compounds, and the integron-integrase gene intI1. In contrast, sodium silicate dosage at 10 mg/L resulted in decreased bacterial growth and antibiotic resistance selection compared to the other corrosion inhibitor additions. Source water collected from the drinking water treatment plant intake pipe resulted in less significant changes in ARB and ARG abundance due to corrosion inhibitor addition compared to source water collected from the pier at the recreational beach. In tandem with the antibiotic resistance shifts, significant microbial community composition changes also occurred. Overall, the corrosion inhibitor sodium silicate resulted in the least selection for antibiotic resistance, which suggests it is the preferred corrosion inhibitor option for minimizing antibiotic resistance proliferation in DWDS. However, the selection of an appropriate corrosion inhibitor must also be appropriate for the water chemistry of the system (e.g., pH, alkalinity) to minimize metal leaching first and foremost and to adhere to the lead and copper rule. IMPORTANCE Antibiotic resistance is a growing public health concern across the globe and was recently labeled the silent pandemic. Scientists aim to identify the source of antibiotic resistance and control points to mitigate the spread of antibiotic resistance. Drinking water is a direct exposure route to humans and contains antibiotic-resistant bacteria and associated resistance genes. Corrosion inhibitors are added to prevent metallic pipes in distribution systems from corroding, and the type of corrosion inhibitor selected could also have implications on antibiotic resistance. Indeed, we found that sodium silicate can minimize selection of antibiotic resistance while phosphate-based corrosion inhibitors can promote antibiotic resistance. These findings indicate that sodium silicate is a preferred corrosion inhibitor choice for mitigation of antibiotic resistance.

Comparison of pipe loop and pipe section reactor methods for estimating chloramine decay in harvested distribution system pipes

To assess chloramine decay, this study compared the use of pipe loops, which incorporate continuously flowing water, to static pipe section reactors (PSRs). Unlined cast iron (UCI) and cement-lined ductile iron (CLDI) were harvested from distribution systems. These were directly compared to virgin polyvinyl chloride (PVC) pipe at low (0.03 m/s) and high (0.09 m/s) water velocities as well as hydraulic residence times (HRT) of 6 and 24 h. Pipe material was observed to exert the greatest impact on chloramine decay, followed by flow velocity. First-order decay coefficients obtained using pipe loops were statistically similar to those for PSR trials when considering UCI and CLDI pipe, irrespective of pipe velocity or water age. Overall results suggest that the use of PSRs may serve as a viable and cost-effective alternative to pipe loops for assessing the impact of operational variables on disinfectant decay.

A literature review of bench top and pilot lead corrosion assessment studies

Bench top and pilot lead corrosion studies are gaining more interest, considering revisions and upcoming improvements to the Lead and Copper Rule. This literature review identified studies ranging from simpler month(s)-long bench top dump-and-fill stagnant water tests (coupon tests/standing pipe tests) to more complicated year(s)-long intermittent flow pilot studies (recirculating pipe loops/once through pipe rigs). With increasing complexity in design and operation, studies more closely approximated real plumbing conditions (e.g., by incorporating harvested lead pipes and intermittent flow regimes) at increased cost, footprint, and duration. Comparison of bench top and pilot designs (in terms of lead test piece age/dimensions/configuration/replicates, study duration, sample collection, and other factors) can assist drinking water utilities, consultants, academics, and others to select a design that matches their needs and constraints. No matter the choice, surrogate systems cannot replace actual system water testing and are best complemented by other corrosion assessment tools.

Early phase effects of silicate and orthophosphate on lead (Pb) corrosion scale development and Pb release

Orthophosphate is widely used to control lead (Pb) release in drinking water distribution systems, but phosphorus addition is not sustainable. Alternative corrosion control treatments are needed, and sodium silicate is one possibility. Here, pre-corroded Pb coupons-with and without free chlorine-were used to examine early-phase corrosion scale development after silicate addition, with orthophosphate as a reference corrosion inhibitor. Scale development was evaluated in terms of total Pb release, phase transformation, electrochemical impedance, morphological changes, Pb dissolution kinetics, and short-term Pb-Cu galvanic corrosion. Elevated Pb release occurred for approximately one month after silicate addition, and total Pb release peaked at 1968.1 μg/L and 1176.9 μg/L from systems with and without free chlorine, respectively. In contrast, orthophosphate-treated coupons exhibited fewer, less pronounced spikes in Pb release. By day 354, the median total Pb release from orthophosphate-treated coupons with and without free chlorine had decreased to 3.7 and 5.0 μg/L, respectively, while the median total Pb release from corresponding silicate-treated coupons was much higher, at 44.9 μg/L and 34.3 μg/L. Calcium lead apatite (Ca_0.56Pb_3.77(PO₄)₃OH_0.67) was identified in orthophosphate-treated scales, with hydroxylpyromorphite (Pb₅(PO₄)₃OH) present in the absence of free chlorine. Plattnerite occurred on coupons in all chlorinated systems. Pb silicate compounds were not detected, but Ca₂SiO₄ and Na₂Ca₂(SiO₃)₃ were identified by X-ray powder diffraction. The charge transfer: film resistance ratio characterizing the orthophosphate-treated coupons decreased slowly while that of the silicate-treated coupons increased after silicate was added. These variations suggest orthophosphate provided better corrosion control than silicate did. Silicate treatment generally caused degradation of the top Pb scale layer, resulting in elevated Pb release, while orthophosphate encouraged the growth of more structured, generally thicker, corrosion scales that were effective in limiting Pb release.

Seasonal Lead Release into Drinking Water and the Effect of Aluminum

Monitoring lead in drinking water is important for public health, but seasonality in lead concentrations can bias monitoring programs if it is not understood and accounted for. Here, we describe an apparent seasonal pattern in lead release into orthophosphate-treated drinking water, identified through point-of-use sampling at sites in Halifax, Canada, with various sources of lead. Using a generalized additive model, we extracted the seasonally varying components of time series representing a suite of water quality parameters and we identified aluminum as a correlate of lead. To investigate aluminum's role in lead release, we modeled the effect of variscite (AlPO₄·2H₂O) precipitation on lead solubility, and we evaluated the effects of aluminum, temperature, and orthophosphate concentration on lead release from new lead coupons. At environmentally relevant aluminum and orthophosphate concentrations, variscite precipitation increased predicted lead solubility by decreasing available orthophosphate. Increasing the aluminum concentration from 20 to 500 μg L^-1 increased lead release from coupons by 41% and modified the effect of orthophosphate, rendering it less effective. We attributed this to a decrease in the concentration of soluble (<0.45 μm) phosphorus with increasing aluminum and an accompanying increase in particulate lead and phosphorus (>0.45 μm).

Initial Formation and Accumulation of Manganese Deposits in Drinking Water Pipes: Investigating the Role of Microbial-Mediated Processes

Microbial Mn(II) oxidation occurs in areas with insufficient disinfectants in drinking water distribution systems. However, the overall processes of microbial-mediated Mn deposit formation are unclear. This research investigated the initial Mn(II) oxidation, deposit accumulation, and biofilm development in pipe loops fed with nondisinfected finished water for 300 days. The results show that it took 20 days for microbial Mn(II) oxidation and deposition to be initiated visibly in new pipes continuously receiving 100 μg/L Mn(II). Once started, the deposit accumulation accelerated. A pseudo-first-order kinetic model could simulate the disappearance of Mn(II) in well-mixed pipe loop water. The observed rate constant reached 2.81 h^-1 [corresponding to a Mn(II) half-life of 0.25 h] after 136 days of operation. Without oxygen, Mn(II) in the water also decreased rapidly to 1.0 μg/L through adsorption to deposits, indicating that after the initial microbial formation of MnO_x, subsequent MnO_x accumulation was attributable to a combination of microbial and physicochemical processes. Compared to the no-Mn condition, Mn(II) input resulted in 1 order of magnitude increase in biofilm formation. This study sheds light on the increasingly rapid processes of Mn accumulation on the inner surfaces of water pipes resulting from the biological activity of Mn(II)-oxidizing biofilms and the build-up of MnO_x with strong adsorption capacity.

Pilot-scale comparison of sodium silicates, orthophosphate and pH adjustment to reduce lead release from lead service lines

Sodium silicate is thought to mitigate lead release via two mechanisms: by increasing pH and by forming a protective silica film. A pilot-scale study using an excavated lead service line (LSL) fed with water from a Great Lakes source was undertaken to: (1) clearly distinguish the pH effect and the silica effect; (2) compare sodium silicate to orthophosphate and pH adjustment; (3) determine the nature of silica accumulation in the pipe scale. The LSL was cut into segments and acclimated with water at pH 7.1. Median dissolved lead was 197 µg/L in the last 8 weeks of acclimation and dropped to 16 µg/L, 54 µg/L, and 85 µg/L following treatment with orthophosphate (dose: 2.6 mg-PO₄/L, pH: 7.9), pH adjustment (pH: 7.9) and sodium silicate (dose: 20 mg-SiO₂/L, pH: 7.9), respectively. When silica dose was increased from 20 mg-SiO₂/L to 25 mg-SiO₂/L (pH: 8.1), lead release destabilized and increased (median dissolved lead: 141 µg/L) due to formation of colloidal dispersions composed mainly of lead- and aluminum-rich phases as detected by field flow fractionation used with inductively coupled plasma mass spectrometry. Si was present in the scale at a maximum of 2.2 atomic % after 17 weeks of silica dosing at 20 mg- SiO₂/L. Under the conditions tested, sodium silicate did not offer any benefits for reducing lead release from this LSL other than increasing pH. However, sodium silicate resulted in lower levels of biofilm accumulation on pipe walls, as measured by heterotrophic plate counts, when compared to orthophosphate.