3.1. pH and TS of Solutions
Table 1 shows the effect of activated carbon on the pH and TS of LaP1 solutions. The pH of the seven stages was 11.64, 11.61, 11.51, 11.32, 11.05, 10.91, and 10.86, respectively. The pH of solutions slightly decreased (
p < 0.05) with an increase in the number of washings. A significant difference was detected between the fourth, fifth, sixth, and seventh stages and the diluted solution before washing with activated carbon (11.72). The pH of the solutions has a critical effect on the decolorization solution using the activated carbon technique. A study on sugar syrup shows that a pH between 4.5 and 6.7 led to easy decolorization compared to acidic and alkali conditions [
38]. As the solutions were in alkali conditions, this could lead to activated carbon treatment having poor efficiency. This could be related to the hydroxide ions in the solutions, which are competitive ions during color removal [
39]. The pH adjustment of solutions before decolorization is recommended to improve the efficiency of activated carbon [
40]. However, this adjustment of pH could affect the structure of LP.
The TS of the bottom layer solution was 27.92, which was dilated with distilled water with a ratio of 1:2.2 to obtain 12.61. The TS of the seven stages of washings were 8.43, 7.78, 6.89, 6.11, 5.91, 5.33, and 6.03%, respectively. The TS slightly decreased with an increase in the number of washings, except in stage number 7, which was slightly higher than stage number 6. No significant difference (
p > 0.05) was found in the TS of LaP1 between the fourth, fifth, sixth, and seventh stages. Comparing the TS of the diluted solution with the final washing stage, it was significantly higher (
p < 0.05). The loss of TS is related to removing the organic compounds from the solution by the adsorbent technique of activated carbon. It is not entirely clear how the adsorption of organic molecules on activated carbon in an aqueous media. Weber Jr and Morris [
41] theorized that intraparticle diffusion is the critical step in the adsorption of several organic compounds on carbon. In a turbulent environment, several mechanisms are involved in the adsorption of organic compounds on activated carbon.
Table 2 presents the pH and TS of LaP2 solutions before and after being treated with activated carbon. The pH of LaP2 solutions was slightly decreased with the increasing number of washings. The pH of the seven stages was 11.48, 11.29, 11.17, 10.89, 10.35, 10.34, and 10.37, respectively. A significant difference (
p < 0.05) was detected between the fifth, sixth, and seventh washing stages and the solution after being diluted with distilled water (11.54). This is related to the removal of organic compounds by adsorption techniques, leading to a decreased solution pH [
42]. Similar results were found in Lubis et al. [
43]. They purified water using activated carbon from natural sources. They found a decrease in water pH after being treated with activated carbon.
Increased ionization, solubility, and hydrophilicity are typically caused by higher pH [
44]. The adsorption of organic compounds was also discovered to be affected by pH changes by modifying the adsorbents’ surface properties and the adsorbate molecules’ electronic properties in activated carbon [
45,
46]. Additionally, it has been demonstrated that the solution’s pH, which affects the charge density of the activated carbon, significantly impacts the adsorption rate [
47]. Mohan et al. [
48] studied the commercial activated carbon’s ability to be adsorbed in an aqueous phase. The results showed that the adsorption of activated carbon is highly dependent on the pH of the solution; as pH rises from 2.0 to 10.5, adsorption capacity decreases.
The TS of the first, second, third, fourth, fifth, sixth, and seventh stages of washings were 8.17, 6.78, 6.21, 5.73, 5.76, 5.48, and 4.93%, respectively. TS slightly decreased with an increase in the number of washings. The solution of the first wash was significantly higher (
p < 0.05) compared to the second, third, fourth, fifth, sixth, and seventh stages. However, there was no significant difference between the second, third, fourth, fifth, sixth, and seventh stages. It appears that the TS of both solutions decreased after being treated with activated carbon. Because of its non-polar nature, activated carbon is notably well-recognized for its ability to adsorb organic compounds [
49]. It is effective in adsorbing different types of chemicals. This led to a decrease in the TS of solutions each time they were treated with activated carbon. Similar results have found that TS decreased for beet sugar after being treated with activated carbon [
50].
3.2. Color Measurements
Table 3 shows the effect of washing numbers with activated carbon on the color parameters of the LaP1 solutions. The standard L*, a*, and b* were 89.53, −4.97, and 6.07, respectively. On the other hand, the L*, a*, and b* of the bottom layer of LaP1 were 33.49, 2.17, and −1.85, respectively. In terms of the first stage, it appears that the L* of the solution was significantly lower (
p < 0.05) than in the seventh stage. The sixth stage (75.31) was slightly higher than the seventh stage (75.04), but there was no significant difference between those two stages. However, the a* of the first wash solution was significantly higher (
p < 0.05) than the seventh wash. It slightly decreased until the fourth wash and then elevated with an increasing number of washings. Furthermore, the b* of the first stage was significantly higher (
p < 0.05) than in other stages, which was expected. The a* and b* decreased as the number of treated solutions with activated carbon increased; however, L* increased on the other side. Januszewicz et al. [
51] found similar results when treating wastewater with activated carbon. Januszewicz’s study reported that the color was slightly removed from the water when it was washed multiple times with activated carbon, and this is due to the high surface area of adsorption.
Table 4 illustrates the impact of the washing stages with activated carbon on the color of LaP2 solutions. It appears that the L* of the solution of the first wash (73.68) was significantly lower (
p < 0.05) than in other washes (81.34 in the second wash vs. 90.21 in the seventh wash). However, the a* of the solution of the first wash (−0.41) was significantly higher (
p < 0.05) than the seventh wash (−5.02). It decreased until the fourth stage (−6.34), then slightly increased. Furthermore, the b* significantly dropped (
p < 0.05) from 45.29 in the first wash to 6.17 in the seventh wash when the solution was washed with activated carbon. When we compared the first stage with the seventh stage of both solutions, the lightness increased, and the redness and yellowness decreased in stage 7. It means that with an increase in the number of washes, the dark color slightly decreased, which was expected. A study was carried out on removing the color of wastewater using activated carbon at different ratios [
52]. The study shows that the efficiency of color removal increased by increasing the dosage of adsorbents.
Table 5 and
Table 6 present the color removal efficiencies of activated carbons with seven washes for both LaP1 and LaP2 solutions. It was clear that depending on the wash of activated carbon, the color removal efficiency varied widely among the solutions. Complete color removal was achieved with seven stages. ∆E of LaP1 solutions was 46.03, 31.13, 16.20, 10.96, 9.27, 14.35, and 14.54 for the first, second, third, fourth, fifth, sixth, and seventh stages, respectively. ∆E was slightly decreased with an increase in the number of washings with activated carbon. The fourth and fifth stages were slightly lower than the sixth and seventh stages. This could relate to the a* parameter being slightly lower (
p 0.05) in those stages. ∆E of the first wash was three times higher compared to the seventh stage. The bottom layer of solutions was 57.04, which is higher than that of the other stages. On the other hand, ∆E of LaP2 solutions was 42.57, 19.45, 10.43, 8.76, 5.28, 1.40, and 0.68 for the first, second, third, fourth, fifth, sixth, and seventh stages of washing, respectively. ∆E was slightly decreased with an increase in the number of washings. The LP decolorization performances of activated carbon are shown in
Figure 1 and
Figure 2. The dark color of both solutions was slightly decreased after multiple washes with activated carbon. The color of solutions tended to be brownish (first and second stages), yellowish (third and fourth stages), clear (fifth and sixth stages), and transparent (colorless) solutions in the final stages.
The numbered pictures represent various doses of activated carbon: 1st = lactose solutions after washing 1 time with activated carbon; 2nd = lactose solutions after washing 2 times with activated carbon; 3rd = lactose solution after washing 3 times with activated carbon; 4th = lactose solutions after washing 4 times with activated carbon; 5th stage = lactose solutions after washing 5 times with activated carbon; 6th = lactose solutions after washing 6 times with activated carbon; 7th = lactose solutions after washing 7 times with activated carbon; bottom layer = the bottom layer of lactose solution after the phosphorylation process; dilated solution = the bottom layer of lactose after the phosphorylation process and being dilated with distilled water with a ratio of 1:2.2.
The numbered pictures represent various doses of activated carbon: 1st = milk permeate solutions after washing 1 time with activated carbon; 2nd = milk permeate solutions after washing 2 times with activated carbon; 3rd = milk permeate solutions after washing 3 times with activated carbon; 4th = milk permeate solutions after washing 4 times with activated carbon; 5th = milk permeate solutions after washing 5 times with activated carbon; 6th = milk permeate solutions after washing 6 times with activated carbon; 7th = milk permeate solutions after washing 7 times with activated carbon; bottom layer = the bottom layer of milk permeate solution after the phosphorylation process; dilated solution = the bottom layer of milk permeate solution after the phosphorylation process and being dilated with distilled water with a ratio of 1:2.2.