Abstract
Certain possibilities of synthesising new water-soluble polymers based on chemical industry waste and local raw materials are considered. By using the latest methods of analysis, the composition, structure, and formation of the polymers are identified, and the possibilities of using the developed polymers to stabilise soils and subsoils and sands of the Aral Sea coastal region are shown.
The drying up of the Aral Sea represents a current global problem. It is made worse by the fact that the shifting sands of the dry Aral bed are heavily saline and contain a vast amount of different harmful chemical agents that can enter the composition of different mineral fertilisers and dust. A serious factor in the deterioration of the ecological conditions in the Aral Sea region is the drift of salts and dust from the territory of these regions [1].
In this context, stabilising the saline sands of the dry Aral bed and creating strong surface structures that do not prevent the growth of plants and that protect against wind erosion by strong aerodynamic flow are among the most urgent problems facing modern polymer chemistry and ecology as a whole [2].
It is known that the dried bed of the Aral Sea is covered with a layer of shifting saline sands with an area of over 2 400 000 ha. The content in them of water-resistant macrostructures of over 0.25 mm, which are of importance for the cultivation of salt-resistant plants on these sands, is low and often amounts to no more than 5–7% of the total mass of the sand, on account of which their efficient use in the agricultural sector of the economy is difficult. In view of this, stabilising the sands against wind erosion by creating a strong surface skin that will ensure anchoring of the mineral particles and salts at the points of their formation with the aim of preventing erosion is an important problem [3].
In this respect, the aim of the research work that we have recently conducted has been to protect shifting sands against wind erosion by chemical anchoring using high-molecular-weight composite additives based on industrial waste of chemical enterprises in Uzbekistan.
In this regard, we have conducted investigations into the synthesis and development of technology for the production of water-soluble polymers based on methacryloyl chloride (MAC) with phosphorus-containing compounds based on waste from the Ammofos-Makham enterprise, as it is known from the literature that MAC readily enters into electrophilic substitution with electropositive centres such as nitrogen and phosphorus. This predetermined the possibility of investigating the behaviour of MAC in reactions of electrophilic substitution with the above compounds, with the aim of producing high-molecular-weight compounds and polyols, and the possibility of using them as structure-forming agents for soil and as stabilisers for sands.
It was found that, when MAC is mixed with phosphorous acid, both in bulk and in organic solvents in a wide temperature range, high-molecular-weight substances are formed that do not contain free molecules of the monomers, i.e. irreversible polycondensation occurs.
The laws governing the polycondensation of MAC with phosphorous acid were studied with equimolar ratios of the initial components in the temperature range 333–373 K for 300 min. The progress of polycondensation was monitored by potentiometric titration of acid groups. As change in reduced viscosity and release of hydrogen chloride are direct results of the described processes, it follows that quantitative assessment of these two factors has also served as a method for determining the polycondensation rate of MAC and phosphorous acid.
To clarify the nature of interaction of phosphorous acid with the above monomer, UV and IR spectra of the initial and end products were investigated, and also proton magnetic resonance (PMR) spectra of the initial compounds.
IR spectroscopic study established the existence of absorption at frequencies of 760–730, 1100, 1400, 1500, and 1965 cm–1, characteristic of C–O–P bonds, and also stretching vibration of the hydroxyl groups at frequencies of 2500 and 3020 cm–1. It was also established that in the IR spectrum of the polymer, obtained on the basis of the interaction of phosphorous acid with MAC, the band corresponding to stretching vibrations of the C–Cl bond is displaced into the low-frequency region to 1350 cm–1, by comparison with that in the spectrum of the waste. The stretching vibration of the C–Cl bond (850–800 cm–1) relating to the MAC group disappears on account of the formation of a new chemical OH bond in the 2500 and 3020 cm–1 region. Here, new intense absorption bands are also formed in the 1050–1100 cm–1 region, relating to asymmetric vibrations of the ester bond (–C–O–P–) (1250, 930 cm–1) in the process of interaction of MAC with phosphorous acid. The results of IR, PMR, and UV spectroscopic investigations, elemental analysis, and also potentiometric titration indicate that the obtained product is a linear polymer.
The reaction products comprise very viscous uncoloured or amber-coloured liquids with a specific odour; their physicochemical characteristics have been fully identified.
We then investigated the application properties of the developed polymer as a structure-forming agent for soils and sands. The investigation was conducted on specimens of shifting saline sands from the dry bed of the Aral Sea. Investigation of the stabilisation of these sands with high-molecular-weight additives using polymer sand binders was carried out with concentrations of the solutions of 0.1, 0.3, 0.5, and 1.0%. Surface treatment of the sand was done by spraying the surface with polymer solutions.
In the course of the investigation it was established that the interaction of the water-soluble polymer (WSP) developed by the present authors with dispersed particles depends on many factors: the concentration of the WSP and of the mineral suspension, the presence of electrolytes, the temperature, the salt content, etc. Of the mineral suspensions, a systematic and detailed study was conducted on soil and clay suspensions from the Aral Sea coastal region. Thus, the influence of the developed WSP, by analogy with polyacrylamide (PAA), showed that polyacrylamide compounds interact with soil particles, on account of which structure formation occurs in the suspension. The pH of the soil suspension in the presence of these polymers does not change, just as in suspensions with gelatine (
The influence of the concentration of the polymer solution on the volume of residue, the filtration rate, and the viscosity of the filtrates of the soil suspension
The relative volume of residue in the soil suspension under the influence of the polymers synthesised by us changes in the same way. However, in the case of industrial polymer PAA, there is more soil suspension (
The change in the filtration rate of a soil suspension on the gelatine concentration: 1 – initial soil; 2 – 0.2% gelatine; 3 – 0.5% gelatine. Vertical axis: V, mL; Horizontal axis: τ, min
The change in the filtration rate of a soil suspension on the WSP concentration: 1 – initial soil; 2 – 0.05% WSP; 3 – 0.1% WSP; 4 – 0.3% WSP. Vertical axis: V, mL; Horizontal axis: τ, min
This is due to the fact that, under the action of a polyelectrolyte, firstly peptisation of the soil particles, and consequently the blockage of pores by finer particles, may occur, and secondly the surface of particles may be screened by the polymer. On account of this, favourable conditions are created for the sliding of particles relative to each other, and for the emergence of dense packing, which retards the passage of liquid phase throught the layer of residue. However, if peptisation were to have occurred, the volume of residue should have decreased continually and the dispersed phase would have been cloudy, which was not the case.
The reduction in the specific viscosity of the soil filtrate that was observed in experiments, by comparison with the initial gelatine solutions, confirms the correctness of the second assumption – envelopment of the surface of the soil particles by the polymer. The maximum adsorption of gelatine on soil particles was calculated to be 8.2%, which is significantly greater than for PAA.
In this way, polyacrylamide polymers and gelatine enter into reaction with the soil particles. Depending on the nature of the polymer, this leads either to an increase in sliding of the enveloped soil particles relative to each other or to completion of structure formation of the soil particles.
In an investigation of samples from the bed of the dry Aral Sea it was established that the filtration rate in the presence of the WSP developed by us (in all indicated concentrations) increases, but not in proportion to the increase in its dose, because in the process of interaction of soil particles with the polymer, aggregates of different size are produced. On a light-coloured sample, increase in the filtration rate is even more pronounced than on a non-saline light-coloured sample. On a light-coloured saline sample, for comparison with the WSPs developed by us, we investigated polymer PAA, which in the same sequence increased the filtration rate of water through the soil.
The grain size distribution of the soil affects the process of binding of microaggregates. Against a background of calcium chloride, soil particles of different size form the loosest residues in the presence of the WSP.
Analysing the produced experimental data, we calculated the effectiveness of WSPs in terms of their influence on soil suspensions. It turned out that, in typical soil and subsoil, the effectiveness of the WSP in a dose of 0.05 wt% of the soil is equal to 11, while the effectiveness of PAA is 9.
From the above it follows that, under the influence of the WSP developed by us from waste and local raw resources, structure formation in 10% soil suspensions occurs, as a result of which larger aggregates are formed, promoting an increase in the rate at which the liquid phase passes through the layer of residue. Structure formation leads to a reduction in the aggregate stability of the suspension, which in turn is connected with a reduction in the degree of dispersion of the system.
By means of sedimentation analysis, the effect of the WPS synthesised by us on the degree of dispersion of different mineral suspensions was studied. Tests were conducted with 0.2% suspensions, to which, 24 h after preparation, polymer solutions were added in the appropriate doses. The kinetic effects were studied after 5 min, 10 min, 15 min, etc. It was established that, under the influence of the WSP, there is a change in the degree of dispersion and in other quantities associated with the particle size of the suspension. Equilibrium is established after only 5 min contact of the suspension with the polymer. Irrespective of the type of mineral system, the radius of the most probable particles increases, which indicates aggregation of the particles of dispersed phase under the influence of the polymer additive. Aggregation of the dispersed phase occurs within the range of the optimum polymer dose. Subsequent increase in the WSP concentration promotes an increase in the degree of dispersion and accordingly a reduction in the magnitude of the most probable particle radius, which may cause the decomposition of the formed aggregates under the influence of the polyelectrolyte and stabilisation of the suspension particles.
In this way, the structure formation in mineral suspensions under the influence of the WSP depends in a complex manner on the polymer concentration.
Study of the change in filtration properties of a typical irrigated sierozem and light-coloured sierozem under the influence of the polymers developed by us showed that, on a typical irrigated sierozem, with the addition of the polymer in doses of 0.005–0.3 wt% of the soil sample, the filtration rate increases with increase in the polymer concentration. Both in the pasty state and in the form of dry powder, the polymer accelerates filtration, but to a lesser extent than PAA.
The results of investigations of the effect of high-molecular-weight composites on the formation of wind- and water-resistant aggregates, and also on the mechanical strength of the skin, showed that the polymer composites developed by us create largely favourable conditions for the cultivation of salt-resistant plants on the stabilised sands of the dry Aral Sea bed.