Arsenic

Arsenic

Goals:

-Devise methods to conduct research safely and to ensure safe disposal of arsenic contaminated waste.

-Determine the best way to prepare and to measure very low concentrations of arsenic.

-Design a reactor system and data collection system that will make it possible for us to begin to optimize treatment processes for efficient and reliable arsenic removal.

-Determine how to create a raw water for testing. Should the raw water be created from distilled water or from tap water? What should be added to the raw water to set the ionic composition? How should pH be controlled?

The processes of flocculation, floc blanket formation, plate settler sedimentation, and filtration are expected to remove As (V). The removal of arsenic by precipitation is expected to be limited by the transport of arsenic to the solid surface of the coagulating agent (either iron or aluminum salts). The flocculation process for groundwater containing arsenic is expected to be inefficient due to the low floc volume fraction. To compensate for the low floc volume fraction it may be necessary to use a longer residence time. Loss of coagulant to the walls of the reactor will also likely be a major problem for small scale reactors given the low solid surface area in suspension. It may be advantageous to use a contact chamber for rapid mix and initial precipitation to reduce losses to the reactor walls.

If this is the case, then a floc blanket consisting of precipitated coagulant could be an efficient reactor for arsenic removal.

Current Research

An important first task is to develop a reliable and sensitive measure for dissolved As (atomic absorption using graphite furnace atomization may be one option if we have a suitable lamp, inductively coupled plasma may be another option if you can get free help from someone with this instrument). There are some wet chemical methods (see Hach), but their sensitivity may not be adequate. Without an analytical tool we won't be able to verify the efficacy of any treatment scheme tested. Measuring dissolved As will require a phase separation from As adsorbed to suspended solids that does not alter the dissolved As concentration. Centrifugation is a good option (filtration probably will not work well). Uptake of dissolved As (adsorption) onto solids will likely depend on pH and ionic strength (as well as the concentration of suspended solids and dissolved As). pH control will be particularly important and it will be difficult to reproduce results if this is not part of the experimental protocol.

In Spring 2013, we determined that aluminum coagulants may be just as effective as iron salts for arsenic removal. We also have researched the lab safety protocols for dealing with arsenic in the AguaClara lab, and believe we can begin testing coagulants next semester. Below is a list of questions we suggest next semester's team continue researching. We've also included a task list in our final report detailing how to get the lab arsenic ready and a potential experimental setup to test arsenic coagulation.

In Fall 2013, we began laboratory testing. The team initially began by preparing the lab area for hazardous chemicals with the proper personal protective equipment and appropriate signage. Then we went on to collect testing equipment such as the tube tumbler, test tubes, and the appropriate chemicals. We had to order tubes which had a filter insert so that we could test our small scale filtration process. There were also several chemicals which we needed to purchase to create our test groundwater. Once all of our materials were collected we were able to begin testing our process of sedimentation and filtration in a small scale reactor. Our GFAAS machine is not fully operational at the time; there are a few tubes which need repairing, but once this is complete, we are confident that we will be able to read concentrations of arsenic which are below five parts per billion.

Future Research

• Which coagulant, Fe(Cl)3, alum, or PACl is better at removing arsenic?
• Is arsenic removal limited by the mass transfer of arsenic to a precipitated coagulant surface, or by capture of the precipitated coagulant by plate settlers and filters?
• Would a floc blanket formed from coagulant precipitate enhance arsenic removal?
• Does addition of a small amount of clay enhance flocculation and arsenic removal?
• How can we reduce the amount of coagulant loss to the reactor walls? (contact chamber, clay)
• What is arsenic removal correlated with? What is the limiting factor?
• Due to the geometry of arsenic flocs, should we consider a coagulation-filtration system rather than attempting flocculation and sedimentation?