About the author:
Rick Bacon is CEO for Aqua Metrology Systems. Bacon can be reached at [email protected] or 408.523.1900.
New data on the efficacy of stannous chloride (SnCl2) to treat hexavalent chromium [chromium-6, or Cr(VI)] released in Fall 2016 highlighted the methodology as an economical treatment approach with the potential to significantly reduce the capital, operations and maintenance costs of chromium remediation compared to best available technologies, which have been surrounded by controversy over their high capital and operating costs.
Research has shown the reduction of Cr(VI) to Cr(III) by both stannous chloride and ferrous ions to be highly effective. However, conventional stannous reagent dosing methodology is not without disadvantages. Stannous salt solutions are highly corrosive, toxic and hazardous reagents requiring special shipping, storage and handling. The reagents have a limited shelf life due to their chemical instability, and once expired, they must be disposed of safely. Due to the high density, viscosity and acidity of stannous salt solutions, they also may require a complex delivery system design and control. All combined, conventional dosing approaches for stannous reagents are associated with high capital and operational costs.
Aqua Metrology Systems (AMS) has supported the evaluation of SnCl2 as a potential lower-cost chemical reagent for treating Cr(VI) contamination through studies at remediation sites. AMS believes that certain characteristics of the traditional SnCl2 reagent make it unsuitable for treating Cr(VI) in a consistent and predictable manner.
As a result, AMS designed the patent-pending SafeGuard H2O, an intelligent online Cr(VI) treatment system that generates a stannous ion reagent in-situ via an electrolytic process. The system also features real-time sensing to ensure that the performance is optimized to avoid under- or over-treatment, and that any deterioration in system performance is signaled to permit timely remedial intervention. A two-week demonstration of the technology was undertaken at the city of Los Banos, Calif., from December 11 to 22, 2017.
Demonstration Study
A SafeGuard H2O demonstration unit was installed at Los Banos Well #14, one of the city’s wells that has elevated Cr(VI) levels of 40 ppb and extremely challenging water quality due to the presence of high levels of uranium, high conductivity and hardness (Table 1). This type of water composition is particularly problematic for traditional Cr(VI) treatment systems based on ion exchange or zerovalent Cr(VI) remediation, and excellent for detailing the treatment efficacy of the SafeGuard H2O system.
During the two-week evaluation period, the SafeGuard H2O system’s performance for consistently treating Cr(VI) below 10 ppb under extremely challenging water conditions was evaluated.
Online Cr(VI) Remediation System
In the standard process design of the SafeGuard H2O system, a stannous ion reagent generator is installed inline with the reagent generation stream and controlled by an automated galvanostat (amperostat). The galvanostat maintains a certain electric current sufficient to generate a stannous ion reagent into the stream at targeted levels, according to incoming Cr(VI) contaminant levels and flow rate. A stannous ion reagent is continuously produced and delivered by water flow to the main water stream, where it is mixed and further reacts with chromate, reducing it into Cr(III) in the contactor vessel. An online Cr/tin monitoring system continuously analyzes tin and chromate levels at critical process steps and reports the results to the main control system every thirty minutes. Tin dosing is adjusted according to real-time process data, ensuring a high level of system automation and integrity. Treated water proceeds to the storage tank, where it can be either incorporated into a water-blending scheme or discharged for public consumption. A standard SafeGuard H2O system design process flow diagram is detailed in Figure 1, pg. 35.
Hexavalent-Trivalent Chromium Conversion Reaction Efficiency
Raw well water was continuously treated by
in-situ generated stannous reagent (1.0 ppm dose). Six samples of treated water were collected on December 11 and 13, 2017, and immediately analyzed for Cr(VI) using the online SafeGuard monitor at site within 5 to 6 minutes after sampling (SG 0 h) and then again 24 hours later at AMS’s lab (SG 24 h) on another SafeGuard Cr(VI) monitor .
One representative sample from each daily series was split, preserved and delivered to a third-party certified lab (BC Labs) for further Cr(VI) analysis. The residual Cr(VI) results from the 12 field samples were analyzed by the online SafeGuard Monitor (SG) monitor and BC Labs.
Immediately after sample collection (SG 0 h Data), hexavalent chromium residuals levels were 2 to 3 ppb. However, within 24 hours, Cr(VI) levels decreased to 1 to 2 ppb (SG 24 h Data). Hexavalent chromium results obtained in two split samples by the third-party certified laboratory (BC Labs) were under the detection range (below 0.2 ppb).
Some additional decrease in residual hexavalent chromium levels in treated raw samples indicates ongoing reduction process after sampling. Low residual hexavalent chromium levels detected by the SafeGuard monitor as well as BC Labs indicates a high hexavalent chromium conversion efficiency.
Hexavalent-Trivalent Chromium Conversion Reaction Kinetics Study
Hexavalent chromium conversion kinetics (rate) is represented as fraction of converted Cr(VI) related to the initial level in untreated raw water as a function of reaction (contact) time.
The SafeGuard H2O system was set to generate stannous reagent into raw water flow at 1.0 ppm and 0.5 ppm. Treated water samples were collected and analyzed on the online SafeGuard Cr(VI) analyzer at 3-, 10- and 15-minute intervals. The minimal time required for testing on the SafeGuard analyzer is 3 minutes.
The hexavalent chromium conversion reaction rate is fast. At the 1.0 ppm reagent dose rate over 90% of initial Cr(VI) levels can be converted into trivalent form within a 3-minute contact time. At the 0.5 ppm reagent dose rate, approximately 88% of initial Cr(VI) converted into trivalent form during a 3-minute contact time. There was an insignificant effect on conversion reaction at both dosing levels (1.0 ppm and 0.5 ppm) when the contact time was increased to 15 minutes.
The highest reagent generation efficiency and reagent-contaminant reaction rate occurred with a stannous generation rate of 50 to 100 mg per minute at given (3 liters per minute) treated water flow. The SafeGuard H2O system performance under optimal conditions.
At the 1.0 and 0.75 ppm reagent dose rates, the online SafeGuard Cr(VI) monitor detected trace levels of residual hexavalent chromium, while outside lab results were under the detection range. These low residual Cr(VI) contaminant levels indicate a high efficiency (over 90%) of the conversion process from Cr(VI) to the trivalent form.
High kinetics of Cr(VI) conversion reaction were confirmed in less than five minutes from the treated water sample collection to the SafeGuard analyzer displaying results.
Decreasing the reagent dose rate down to
0.5 ppm resulted in a lower conversion efficiency, approximately 90%. At a stannous reagent dose of 0.25 ppm, data from the online SafeGuard analyzer and the outside lab support a conversion efficiency of less than 50%.
Conclusions
The SafeGuard H2O system successfully demonstrated the ability to efficiently generate stannous reagent levels (0.25 to 100 ppm) into raw water stream and to convert hexavalent chromium into trivalent form with high efficiency and stability.
The system’s performance was monitored in real time using the automated SafeGuard Chromium monitor, a feature of the Cr(VI) remediation system that contributed to fast system setup and optimization. Analytical data from the online method and laboratory showed good agreement, further validating the ability of the SafeGuard H2O system to monitor the Cr(VI) remediation process in real time.
SafeGuard H2O’s real-time sensing ensures that performance is optimized to avoid under- or over-treatment, and that any deterioration in system performance is signaled to permit timely remedial intervention.
The experimental results suggest a high probability for the scalability of the SafeGuard H2O system for point-of-supply and point-of-entry systems. Full-scale evaluation is required to further demonstrate the long-term system performance of SafeGuard H2O in field conditions.