ABSTRACT
To explore the migration law of magnesium ions (Mg2+) during freezing and melting processes, laboratory simulation experiments involving freezing and melting were carried out to investigate the influence of ice thickness, freezing temperature, initial concentration, and initial pH on the distribution of Mg2+ in the ice-water system. The distribution coefficient "K" (the ratio of the Mg2+ concentration in the ice layer to the Mg2+ concentration in the water layer under ice) was used to characterize the migration ability of Mg2+. The results showed that during the freezing process, the concentration distribution of Mg2+ in the ice and water two-phase system was as follows: ice layer < water before freezing < water layer under ice; in other words, it migrated from ice layer to the water layer under ice. "K" decreased with increasing ice thickness, freezing temperature, initial concentration, and initial pH; the higher the ice thickness, freezing temperature, initial concentration, and initial pH were, the higher the migration efficiency of Mg2+ into the water layer under ice was. During the melting process, Mg2+ was released in large amounts (50-60%) at the initial stage (0-25%) and in small amounts (25-100%) uniformly in the middle and later periods. According to the change of Mg2+ concentration in ice melt water, an exponential model was established to predict Mg2+ concentration in ice melt period. The migration law of Mg2+during the freezing and melting process was explained by using first principles. Graphical abstract.
Subject(s)
Ice , Magnesium , Freezing , Ions , Water/chemistryABSTRACT
Effects of microfiltration membrane adsorption for eight types of pesticides with diverse physicochemical properties on direct injection liquid chromatography-tandem mass spectrometric method ( DI-LC-MS/MS) were investigated. The results showed that the adsorption effects ( AE,%) on the loss of signal detection at concentration of 2. 5 μg/L ranged from 8%-92%, 0%-81% and 2%-59% for nylon (NYL), polyethersulfone ( PES) and hydrophilic polypropylene ( GHP) membrane, respectively ( accumulated volume of feed is 1 mL). The pesticides with higher octanol-water partition coefficient (lgKow) values appeared to adsorb onto the membrane surface more readily than those with lower lgKow. The AE was found to be alleviated as the feed volume of samples increased to 10 mL, except the case of profenofos adsorption on PES membrane ( AE=9%) . The effects of PES membrane on the eight pesticides at initial individual concentration of 0 . 25 , 2. 5 and 25 μg/L were also studied. The results showed that, the lower the initial concentration of pesticides, the stronger adsorption effect under the same filtration conditions was obtained. Besides, it was effective in eliminating the membrane adsorption effects by adding 40% methanol ( V/V) into water samples as a modifier, which could consequently improve the accuracy of DI-LC-MS/MS.
