The features of methanal sorption by low-basic anion-exchange resin in different initial forms
Carbonyl compounds, in particular methanal, refer to organic toxic contaminants and their removal from the environmental media is an important technological task. The anion-exchange resins containing low-basic functional groups are known as effective sorbents of aliphatic and aromatic aldehydes. This study considers the peculiarities of methanal sorption by a poly-functional anion-exchange resin in the form of free base and in chloride-form at initial aldehyde concentration 0.010 mol/dm3 and higher. The anion-exchange resin Purolite A 830 having primary, secondary and tertiary amino-groups has been used as a sorbent. The kinetic curves of methanal uptake have been measured in static conditions using the procedure of confined volume. The equilibrium attainment time in the system sorbent-sorbate does not depend on the initial form of A 830 and reaches 60 hours. Earlier it was found that for diluted solutions of methanal (C3) this value is 20 times higher. In the diluted solutions the main mechanism of methanal uptake is chemisorption leading to imines formation in sorbent phase. The studied solution concentration increase results in the additional reactions of methanal such as its hydration and partial polymerization. Methanal hydrate dissociates in the basic medium of anion-exchange resin and can be sorbed in this case as anion. All the mentioned reactions slow down the velocity of sorption but the capacity of the anion exchange resin regarding methanal increases. The total capacity for free base form is 4.50 mmol/g and for chloride-form is 2.15 mmol/g. The initial form of the anion-exchange resin determines the impact of chemisorption mechanism in the methanal uptake. The decrease of the anion-exchange resin sorption capacity regarding methanal for chloride-form deals with the decrease of nucleophile character of amino-groups in the salt form. Applying of the Boyd-Adamson formal approach to the kinetic curves has shown that the limiting stage of formaldehyde sorption in the considered range of concentration is internal diffusion.