Mercerization was carried out to improve sample wettability which is associated with effect of sliding and smoothness of cellulose fibers, and to reduce the edge angle of wetting. Moreover, mercerization improved absorptive properties and imparted a greater affinity for dyes and various chemical finishes. After sample impregnation with NaOH followed by washing, its protonation-deprotonation occurred and resulted in surface hydrophilization with an increase in the hydrophilic-lipophilic balance of the sample.
Phosphorylation of mercerizated cotton cellulose was accompanied by both external material and structural changes. The sample had a yellowish color after heating up to 145ºC, the intensity of which was subsequently washed out by hot and cold distilled water. The strength characteristics were noticeably changed. According to the results on tensile strength investigation the heat treatment did not reduce the sample strength characteristics during phosphorylation in the absence of H3PO4. The standard maximum tear force for untreated and phosphorylated sample was observed to be equal 17.2 ± 0.5 and 17.9 ± 0.6 MPa. The difference within the statistical margin of error indicates that there were no changes in the molecular and supramolecular structure after both mercerization and heat treatment of the cotton cellulose sample. However, the strength characteristics of the sample were reduced by almost 2 times (Fig. 1) despite on low H3PO4 concentration (0.20 mol L− 1). Apparently, it was caused by changes in the structure of the molecules and partial destruction which seemed to increase with raising concentration of phosphoric acid in the phosphorylating solution. It should be noticed that the complete destruction of the sample was not observed with H3PO4 concentration up to 2.403 mol L− 1.
The phosphorus content was determined as phosphate ions concentration after wet oxidation of phosphorylated mercerized cotton. The sample wet oxidation was carried out by using a hot concentrated sulfuric acid (98%, 3 volumes) and hydrogen peroxide (60%, 1 volume). After sample complete dissolution and discoloration of the solution, heating was performed to decompose the excess hydrogen peroxide completely. It is known that all phosphorus of the phosphorylated sample transforms into phosphoric acid.
Table 1
Results on phosphorus content determination in the cotton cellulose sample before and after phosphorylation obtained by the spectrophotometric method based on the color intensity of the reduced phosphorus molybdenum complex.
№
|
C(H3PO4), mol L− 1
|
Content P, wt.%
|
n(P) in 1 g textile, mmol g− 1
|
1
|
0
|
0.062 ± 0.006
|
0.0201 ± 0.0020
|
2
|
0.201
|
0.55 ± 0.06
|
0.179 ± 0.018
|
3
|
0.399
|
1.28 ± 0.13
|
0.414 ± 0.041
|
4
|
0.601
|
1.49 ± 0.15
|
0.481 ± 0.048
|
5
|
0.802
|
1.89 ± 0.19
|
0.611 ± 0.061
|
6
|
1.01
|
2.31 ± 0.23
|
0.745 ± 0.075
|
7
|
1.20
|
2.48 ± 0.25
|
0.802 ± 0.080
|
8
|
1.40
|
2.78 ± 0.28
|
0.898 ± 0.090
|
9
|
1.60
|
2.90 ± 0.29
|
0.938 ± 0.094
|
10
|
1.80
|
2.73 ± 0.27
|
0.881 ± 0.088
|
11
|
2.00
|
2.94 ± 0.29
|
0.950 ± 0.095
|
12
|
2.20
|
2.53 ± 0.25
|
0.816 ± 0.082
|
13
|
2.40
|
2.73 ± 0.27
|
0.881 ± 0.088
|
The data shown an obvious increase in phosphorus content in the sample which corresponds to the rising phosphoric acid concentration in the phosphorylating solution. The number of phosphorus-containing groups per 1 g of the sample increased from 0.179 to 0.950 mmol along with phosphoric acid concentration increased from 0.201 to 2.00 mol L− 1. However, a further increase in the acid concentration did not lead to a phosphorus content increase and was constant within the statistical error.
For comparison the unmercerized cotton fabric was phosphorylated and phosphorus content in its was determined. Unmercerized cotton fabric is more hydrophobic material and is poorly impregnated with aqueous solutions. Therefore, phosphorylation process of the fabric is difficult. In Fig. 3 shows the phosphorus content in phosphorylated mercerized and unmercerized fabric versus phosphoric acid concentration in phosphorylating solution. It is seen that the phosphorus content increases as growing of phosphoric acid concentration both for mercerized fabric and unmercerized. But phosphorus content increases unlinear to about 3 wt. % in mercerized and to about 1 wt. % in unmercerized fabric. Mercerized fabric contents phosphorous more 3 times as unmercerized fabric.
The study results of the mercerized cotton cellulose samples using IR spectroscopy are shown in the Fig. 2.
According to Suflet and Lehtonen (Suflet et al. 2006; Lehtonen et al. 2020), the mercerized cotton cellulose had specific IR wavenumbers: O-H groups stretching vibrations at 3400 ÷ 3500 cm− 1, CH2-groups – at 2800 ÷ 2900 cm− 1and C–O–C-elements of glycosidic bonds – at 1166 and 1115 cm− 1. Several additional bands in the IR spectrum of phosphorylated cotton cellulose was obtained. The absorption band at 1720 cm− 1 appeared due to carbonyl groups which probably were arisen from the pyranose ring opening. Presumably, it can indicate a partial oxidation of the sample. Moreover, the absorption band at 1210 cm− 1 and 830 cm− 1 occurs for bonds valence vibrations P = O and P–O–C respectively. The absence of additional bands in the region of 2800 ÷ 2900 cm− 1 and the shoulder at 920 ÷ 1000 cm− 1, coming from P–O–H valence vibrations, indicates the absence of the pointed functional groups. Furthermore, the appropriate potentiometric titration curve could not be provided due to the inflection point absence. This fact could be the evidence that there were no functional phosphorus-containing groups with hydrogen which could be substituted.
It is known that modified phosphorylated cotton cellulose can be used as a heavy toxic metals sorbent (Hokkanen et al. 2014; Sirviö et al. 2016; Guo et al. 2018). The cotton cellulose sorption properties were studied using Cu 2+ sorption from solutions with different phosphoric acid concentrations (pH = 3, t = 20°C) as a model system. The significant increase of a sorption capacity was shown in the presence of phosphorus-containing groups. The adsorption isotherms shape and their non-linearity indicated an intricate mechanism of the adsorption process. Presumably chemosorption occurs on the surface due to complexation along with Cu2+ physical adsorption. The data obtained were described by the Langmuir monomolecular adsorption model, as a Fig. 3. The plateau corresponds to saturation of sorption centers and formation of a monomolecular layer according to the equation:
where SEC – the static exchange capacity, mmol g− 1; SEC∞ – the limit static exchange capacity, mmol g− 1; K – the exchange equilibrium constant – the ratio of sorption rates constants to desorption rates constants, L mmol− 1; C(Cu2+) – copper ions concentration in the solution, mmol L− 1.
However, a further increase in SEC along with an increase in the Cu2+ sorbed concentration indicated the polymolecular nature of sorption. In this case, the branch rise of the sorption isotherm at more than 125 mmol L− 1 of Cu2+ is explained by the additional copper amounts sorption and the appearance of the second and subsequent adsorption layers, as well as occurrence of chemosorption.
Thereby it is appropriate to compare the maximum static exchange capacity of the monomolecular layers SEC∞ of different phosphorylated cotton cellulose samples (Fig. 4).
It is clear that sample phosphorylation with even low concentrations of phosphoric acid turned out to lead to an increase in the maximum sorption capacity. The results соnfirmed that chemical reaction of phosphate groups addition to the monomer units of cotton cellulose was dealt with. In addition, the initial part of the curve shows an increase in the Cu2+ sorption capacity along with an increase in the concentration of H3PO4 in the phosphorylating solution. The SEC∞ maximum (1.48 ± 0.11 mmol g− 1) can be observed with 1.40 mol L− 1 of H3PO4. The maximum sorption capacity was not changed with up to 1.80 mol L− 1 of H3PO4 within statistical error. A further increase in H3PO4 concentration caused a significant decrease in SEC∞. Furthermore, considering a dependence between a Cu2+ sorbed amount and a phosphorus content it can be concluded that a linear correlation exists (Fig. 6). A Cu2+ were coordinated in the ratio n(Cu2+):n(P) = 1:1 with no more than 0.80 mmol g− 1 of phosphorus content. The exchange equilibrium constant for samples with phosphorus content no more 0.80 mmol g− 1 was 0.0803 ± 0.0087 L mmol− 1. For samples with phosphorus content 0.80 mmol g− 1 and more the exchange constant was equal 0.0331 ± 0.0077 L mmol− 1. The efficiency of Cu2+ sorption became higher with more than 0.80 mmol g− 1 of phosphorus content and the ratio n(Cu2+):n (P) = 1:1 did not match. It can be explained by the bidenticity of phosphorus-containing groups and their ability to coordinate more metal ions than in the 1:1 ratio. Such kind of proportionality is typical for complexation reaction and sorption with chemical nature.
The isotherm of uranyl ions sorption was obtained from a sample phosphorylated with 0.802 mol L− 1 H3PO4 solutions (phosphorus content – 1.89 ± 0.19 wt. %; n(P) = 0.611 ± 0.061 mmol g− 1) (Fig. 7).
It is shown that the obtained experimental data are well described by the Langmuir equation. The maximum static exchange capacity (SEC∞) and the exchange equilibrium constant (K) for the uranyl ions sorption by a the phosphorylated textile sample were 0.822 ± 0.014 mmol g− 1 and 1.844 ± 0.11 L mmol− 1. The high values of the exchange constants and the convex shape of the sorption isotherms indicates that the uranyl ions sorption by phosphorylated cellulose samples is highly selectivity.
The dependence of the static exchange capacity UO22+ on the phosphorus content in textile samples is shown in Fig. 8.
The presented results (Fig. 8) shown that increasing of phosphorus content in samples leads to growth of the uranyl-ions capacity. It indicated an increasing of ionogenic groups number in the composition of sorbents when more concentrated solutions of phosphoric acid are used in the phosphorylating. Higher absolute values of static exchange capacity for uranyl ions in comparison with static exchange capacity for copper are apparently associated with the formation of stronger uranium compounds with phosphate groups on cellulose.
The dependence of the distribution coefficients (Kd) for the radionuclides 241Am, 233U and 239Pu on the phosphorus content in the samples of phosphorylated cellulose is shown in Fig. 9.
The presented in Fig. 9 results demonstrated no statistically significant change of radionuclides 241Am, 233U and 239Pu sorption while increasing of the phosphorus content in the textile sorbents. The average values of the partition coefficient (Kd) was for the radionuclides 965 ± 13, 1600 ± 32, and 734 ± 25 ml g− 1, respectively. In the case of 241Am and 233U the observed effect is apparently associated with the fact that these radionuclides in tap water can be in a hydrolyzed forms, i.e. corresponding hydroxides or radiocolloids. Sorption of these forms of radionuclides occurs due to physical adsorption on the sorbent surface and does not depend on the degree of phosphorylation of the cellulose. The absence of the effect of the phosphorus content on the sorption of 239Pu may be due to the high complexing ability of phosphate groups with respect to Pu4+ ions. In this case, almost complete sorption (more than 99%) of microquantities of plutonium from the acidic solution occurs despite change of ionogenic groups concentration at sorbent. It should be noted that physical adsorption of hydrolyzed and polymeric forms of plutonium (which can be formed in 1.0 mol L− 1 HNO3) be able to occur.
According to the results it was concluded that there is a great opportunity to apply mercerized phosphorylated cotton cellulose as a radioactive actinides sorbent due to such a significant static exchange capacity of uranyl ions. This sorbent allows to compact ash into cement matrices and dispose of a radioactive waste.