As we know there are different types of filter membranes available in the market, such as PP filter membranes, PVDF filter membranes, PES filter membranes, Nylon filter membranes, microporous glass fiber membrane filter, and MCE filter membranes, and so on. Also, each type can be subdivided into more specific ones, like from the pore size. Regardless of pore size, it is important to understand that use conditions do affect particle retention. Even filters with a pore size rating can operate under conditions that allow the passage of particles larger than expected. The commonly used membranes are round shapes, combined with the syringe filter or Buchner funnel. Here is a brief introduction to the filter membranes. Today, HAWACH would like to discuss one of the main applications of the microporous membranes-water sample pretreatment.
Workers engaged in water quality analysis should be aware that unless the water sample is to be analyzed immediately, appropriate pretreatment must be performed before the water sample is stored. Pretreatment is mainly determined according to the different requirements of the water sample to be measured, and filtration is a commonly used pretreatment method.
1. The necessity of water sample pretreatment
In unfiltered samples, there is a potential for changes in the chemical speciation distribution of heavy metals in the sample due to interactions between particulate matter and other substances dissolved in the sample. The researchers found that the adsorption-desorption equilibrium time of heavy metals in the mixture of sediment and water is very fast, generally not more than three days, and the maximum adsorption occurs at pH=7.5. After sampling, any change in solution equilibrium, adsorption sites provided by particulate matter will provide pathways for the migration of metal species, and under certain conditions desorption of adsorbed metals is possible.
Usually, for trace element or organic analysis, the particulate matter in the water sample must first be removed by filtration or centrifugation (if the pollutants in the particulate matter are determined, this part of the sample needs to be collected), and then a protective agent is added, and the water sample is placed in a Store in a non-contaminated container and at a suitable temperature to prevent loss, degradation or morphological change of the active ingredient.
High bacterial concentrations accompanied by the presence of sediments also lead to the loss of water-soluble metal species. The growth of bacteria and algae, including photosynthesis and oxidation, will change the content of CO2 in the water sample and thus lead to changes in pH, which often lead to precipitation, changes in chelation or adsorption behavior, and redox of metal ions in solution. effect. Due to the unpredictable nature of bacterial growth and reproduction in stored samples, the earlier post-sampling filtration, the better. If the time is delayed beyond a few hours, the samples are cryopreserved or acidified to inhibit bacterial growth.
2. Selection of test equipment
The 0.45μm microporous membrane can easily distinguish dissolved matter and particulate matter, and the filtrate passing through the filter membrane may also contain 0.1-0.001μm colloidal particles of microorganisms and bacteria and components less than 0.001μm dissolved in water. The 0.45μm membrane can filter out all phytoplankton and most bacteria. Continuous filtration may sometimes cause blockage of the filter membrane, at this time, it is generally necessary to replace a new membrane or use pressure filtration.
When using filter instruments, attention should be paid to the material of the parts of the instrument in contact with the solution, as well as the type of filter (vacuum or pressurized). The use of rubber plugs for glass filters is easy to cause contamination. Generally, a vacuum filtration system using borosilicate glass is selected. Before filtration, the filter equipment should be washed with dilute acid, usually soaked in 1-3 mol/L hydrochloric acid.
The surface of the untreated filter membrane is very easy to adsorb cadmium and lead in water, but when it is used to filter river water, no changes in the concentrations of the above elements are found. The use of untreated membranes to filter mercury-containing samples from seawater samples may result in losses of 10% to 30%. However, with treated glass fiber filtration, mercury losses can be reduced to less than 7%. The general filter membrane is washed with 20 mL of 2mol/L HNO3 before use and then washed with 50 to 100 mL of distilled water. The receiving beaker or Erlenmeyer flask must be rinsed with distilled water with acid. The 10-20 mL of the filtrate collected at the beginning was removed. For filtration of marine deep water samples. The filter membrane is first soaked with dilute nitric acid.
Pressure filtration or vacuum filtration are two commonly used methods. The pressure filtration speed is fast, and it is suitable for filtering river water samples containing a large amount of sediment. If the water sample is filtered with a φ50mm and 0.45μm membrane, the speed is about 100mL/h. Ultrafiltration membrane is usually used for pressure filtration.