Non-stoichiometric interpolyelectrolyte complexes: Promising candidates for protection of soils
Graphical abstract
Introduction
Erosion is an intensively developing process in soil induced both by natural factors and imprudent human activities (Iturri et al., 2016, Lal, 2001, Ochoa-Cueva et al., 2013, Toy et al., 2002, Vanwalleghem et al., 2017, Yang et al., 2003). It is one of the most critical forms of soil degradation (Cerdà et al., 2010, McBratney et al., 2014, Mengistu et al., 2015). By erosion, soil loses small particles that results in removal of the key nutrient components: humus, nitrogen, phosphorus, potassium, etc., from eroded soils (Brevik et al., 2015, Mchunu and Chaplot, 2012, Withers et al., 2015, Zhang et al., 2015, Zuazo and Pleguezuelo, 2008). This problem is most remarkable in developing countries; however the soil degradation processes take place all over the world. Among various methods for stabilizing soil and ground, polymeric formulations are of particular interest (Inbar et al., 2015, Iyengar et al., 2013, Lee et al., 2013, Movahedan et al., 2012, Orts et al., 2000, Rabiee, 2010, Sadeghi et al., 2016, Sepaskhah and Shahabizad, 2010, Sojka et al., 2007, Zezin et al., 2015). They are relatively cheap, large-tonnage, and easy to use (Puoci et al., 2008, Zezin et al., 2015). However, traditional approaches do not provide a long-term protective effect. Water-soluble polymeric binders are quickly removed from soil with rainwater that leads to the loss of the stabilizing effect even at mild precipitation (Chang et al., 2016). Hydrophobic binders cannot be uniformly distributed in soil, shortly concentrate on the soil surface and form a fragile coating.
Both the fundamental research data and practical application of polymeric binders show that an optimal result for protection towards wind erosion can be achieved when the binder consists of both hydrophilic and hydrophobic fragments (blocks) (Izumrudov and Sybachin, 2006, Zezin et al., 2015). The former interact with hydrophilic regions on the surface of soil particles and bind (glue) them. The latter cause the same effect, but towards hydrophobic soil particle regions (Volikov et al., 2016). This leads, first, to a sharp increase in the binding efficiency and a reduction of binder discharge, and, second, to a uniform distribution of the binder in soil and, at the same time, its lower solubility in water.
These requirements are ideally satisfied by using interpolyelectrolyte complexes (IPECs) which can be prepared by mixing aqueous solutions of oppositely charged polyelectrolytes (Izumrudov et al., 2011, Muller, 2014). IPECs actually represent block copolymers with more or less extended (a)hydrophilic, separated cationic or separated anionic, blocks and (b)hydrophobic, mutually neutralized cationic and anionic blocks; their hydrophobic-hydrophilic balance can be easily varied within wide limits by changing polycation/polyanion ratio (Izumrudov and Sybachin, 2006). It is this feature that determines the ability of such constructs to effectively adsorb on different surfaces (Schwarz and Dragan, 2004, Stoll and Chodanowski, 2002). Two approaches have been described for preparing IPEC formulations: a “two-solution” method with sequential deposition of aqueous solutions of oppositely charged polyelectrolytes on the surface (Mikheykin, 2004) and a “single-solution” method when a mixture of two non-interacting polymeric components in 2–5 wt% aqueous-salt solution is applied (Yamada et al., 2015, Zezin et al., 2015). However, it is difficult to cover the surface uniformly with both polymers using the “two-solution” method; therefore adhesive (mechanical) properties of an IPEC composition can hardly be controlled. The “one-solution” method favors salinization of soil that suppresses plant growth. The latter is immaterial when treating territories not currently in use but becomes a key factor when treating agricultural lands.
In the present article, we describe a novel polymeric formulation, a non-stoichiometric interpolyelctrolyte complex (NIPEC) with an excess of an anionic polymer. NIPECs are formed in the presence of minimum salt concentration that practically has no effect on the water-salt balance of soil (Dubin et al., 2012, Izumrudov and Sybachin, 2006, Muller, 2014, Ortega-Ortiz et al., 2010) We show that negatively charged NIPEC is able to bind to cationic colloidal particles and keep the binding even being electrostatically pre-complexed with heavy metal cations. Finally, we give some examples of the use of NIPEC formulation for soil stabilization and discuss the mechanism of the NIPEC stabilizing effect. Taking together, these results make the NIPEC formulations promising for suppressing erosion of soil and ground contaminated by high toxic metals.
Section snippets
Soil samples
A sample of retisol (sod-podzolic soil) was collected in Moscow region (Russia). An upper 10 cm layer of soil was used with the following characteristics: pH 5.8, a moisture content of 3 wt%, ОМ 2 ± 0.05%, СЕС 7.9 ± 0,14 meq/100 g, ЕС at 25 °С 4.7 ± 0.11 mS/сm These parameters correlate with described elsewhere (Sidorova and Borisova, 2014). Then the sample was dried at 100 °C to constant weight, additionally milled and sifted through a 0.25 mm sieve. The sieved sample was fractioned by sequential passing
Polycation-to-polyanion complexation: NIPEC formation
This paper deals with the interaction between oppositely charged linear polyelectrolytes: cationic CAT (Fig. 1a) and anionic ANI (Fig. 1b). The interaction was accompanied by neutralization of the negative charge of ANI macromolecules that was detected via the decrease of EPM of the resulting ANI/CAT complex particles. Fig. 2a shows how the EPM varies with CAT concentration. The CAT-to-ANI complexation led to a decrease in the ANI charge and an overall change from negative to positive charge at
Conclusions
This paper describes electrostatic interaction (complexation) of two oppositely charged linear polyelectrolytes, an anionic poly(acrylic acid) (ANI) and a cationic poly(diallyldimethylammonium chloride) (CAT). In the excess of ANI, the complexation results in formation of non-stoichiometric interpolyelectrolyte complex (NIPEC) which is actually a block copolymer with hydrophilic regions represented by free (unbound) anionic ANI units and hydrophobic fragments of mutually neutralized ANI and CAT
Acknowledgements
This work was supported by International Science and Technology Center project № KR-2093.
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