Species identification
Despite the heterogeneous nature of the tidal residue, four predominant species were identified (Fig. 1). Cymodocea nodosa is a seagrass composed by leaf bundles, which contain 7 to 9 veins and 1 mm diameter rhizomes. It forms sea meadows in southern locations of the Canary Islands, in Confital Bay where the collection beach is located (Pavón-Salas et al. 2000). The small brown yellowish thallus of the brown algae Lobophora variegata (Phaeophyta) allows to identify an erect habit with simple and lobed to lacerate blades. The height ranged from 1.50 to 5 cm, width from 2–6 cm and thickness from 100–130 cm. This alga conforms beach casts phenomenon in Canary Islands (Sangil et al. 2018).
Other abundant specie found was a parenchymatous algae with ribboned and flat axes. The brownish to olive-green thallus was branched dichotomous forming approximating angles of 15° to 35º and was isotomous (equal branches) like anisotomous (unequal branches). The wide of the branches range from 3 to 6 mm and apices were rounded and bilobed. The height of the thallus was up to 15 cm and the thickness ranged around 85–100 µm. These morphological features correspond to the algae Dictyota dichotoma (Phaeophyta) (Bogaert et al. 2020). This specie is widely spread in the North-eastern Atlantic marine ecosystems (Sangil et al. 2018). Another identified specie found to be a red alga (Rhodophyta) with thallus forming large clumps up to 20 cm high constituted of stoloniferous prostrate axes from which arise erect axes up to 2 mm wide. These axes were covered by dense radially arranged branches. The morphology suggests the specie Asparagopsis taxiformis, as it match with the bibliography (Zanolla et al. 2018). This specie was found in a lower amount compared to the others. It is known to form macroalgal communities in Macronesian Islands (Zárate et al. 2020). The marketability of this mixed biomass depends mostly on the periodicity of the depositions (Milledge et al. 2016). In the case of cellulose, as this natural polymer is present in almost all species of macroalgae and marine plants, the only limitation for the extraction is the amount present in each species.
A multistage process for cellulose extraction was carried on. The first NaOH-AQ stage was performed to increases the accessibility towards cellulose, dissolving the lignocellulosic matrix (Liu et al. 2018). In the AHP stage, the removal of lignin and impurities occurs using H2O2; the initial pH, temperature, reaction time and consistency are the most influential factors (García and Vidal 1984). The required pH for optimal operability ranges from 10 to 12 and the optimum temperature for lignin removal ranges from 60 to 80 ºC (Wuorimaa et al. 2006). The use of additives and stabilizers in the AHP process allow to stabilize chromophore groups by removing metals which can act as catalysators. They can also avoid the premature dismutation of peroxide. Magnesium salts have a stabilizing action by absorbing heavy metals from the slurry and meanwhile avoiding carbohydrate depolymerization by stabilization of glycoside bonds (García and Vidal 1984). On the other hand, DTPA sequestrates transition metals that support the decomposition of peroxide during the treatment (Wuorimaa et al. 2006).
Chemical characterization of the cellulosic pulps
The chemical composition of seaweeds varies with species, maturity, geographical location, seasonality, habitats, and environmental growing conditions (Mendis and Kim 2011). Seaweeds and marine plants have a high moisture content normally representing the 63–96% of the wet weight of the algae and it’s related to variations between species and their different thallus architecture (Baghel et al. 2021). The rest of the weight contains the organic matter (carbohydrates, lipids, and proteins) and ashes. ASH fraction is very variable being of 8.72% in C. nodosa (Kolsi et al. 2016), 7.60% in D. dichotoma (Ozgun and Turan 2015), 16.61% in L. variegata (Nithya et al. 2022) and 36-38.80% in A. taxiformis (McDermid and Stuercke 2003). D. dichotoma have a total lipid content under 10%, whereas L. variegata amount is below 5% (Gosch et al. 2012). Carbohydrates proportion in marine plants and seaweeds species vary depending on their classification: Ochrophyta (brown algae), range from 26.86–41.03%, Chlorophyta (green algae) 53.08–67.40% and Rhodophyta (red algae) 57.79–74.11% (Ahmad et al. 2016). Table 2 shows the averages of the two determinations carried out for the chemical characterization. The carbohydrates content represents the major fraction of the organic matter in seaweeds.
Table 2
Chemical characterization of the cellulosic pulps after NaOH-AQ and H2O2 treatments
XT,Xt,XP | ASH (%) | CEL (%) | EBE (%) | HOL (%) | HWS (%) | LIG (%) | YI (%) |
0, 0, 0 | 7.77 | 85.12 | 1.89 | 45.90 | 36.82 | 7.16 | 56.22 |
1, 1, 1 | 7.43 | 88.60 | 1.43 | 55.98 | 30.00 | 5.32 | 44.81 |
-1, 1, 1 | 7.91 | 84.99 | 1.97 | 52.15 | 30.97 | 7.09 | 51.05 |
1, 1, -1 | 7.49 | 88.23 | 1.50 | 43.01 | 41.08 | 6.53 | 61.46 |
-1, 1, -1 | 8.26 | 84.74 | 1.99 | 41.78 | 39.57 | 8.23 | 67.97 |
1, -1, 1 | 7.62 | 87.52 | 1.64 | 53.67 | 30.90 | 6.31 | 45.83 |
-1, -1, 1 | 8.63 | 84.48 | 2.06 | 51.62 | 29.79 | 8.17 | 50.99 |
1, -1, -1 | 7.64 | 86.34 | 1.67 | 42.57 | 40.77 | 6.86 | 62.38 |
-1, -1, -1 | 8.84 | 84.34 | 2.05 | 41.46 | 38.13 | 8.97 | 68.41 |
0, 1, 0 | 7.69 | 86.06 | 1.78 | 46.47 | 37.62 | 6.90 | 56.50 |
0, -1, 0 | 7.88 | 85.03 | 1.94 | 45.66 | 37.42 | 7.35 | 57.06 |
0, 0, 1 | 7.72 | 85.57 | 1.85 | 51.93 | 32.40 | 6.71 | 47.93 |
0, 0, -1 | 7.83 | 85.11 | 1.91 | 43.32 | 39.82 | 6.99 | 63.78 |
1, 0, 0 | 7.51 | 87.91 | 1.56 | 46.19 | 39.36 | 6.06 | 53.88 |
-1, 0, 0 | 8.45 | 84.56 | 2.03 | 42.04 | 38.12 | 8.44 | 58.43 |
Table 2
Chemical characterization of the slurry after NaOH-AQ and H2O2 treatments.
XT,Xt,XP | ASH (%) | CEL (%) | EBE (%) | HOL (%) | HWS (%) | LIG (%) | YI (%) |
0, 0, 0 | 7.77 | 85.12 | 1.89 | 45.90 | 36.82 | 7.16 | 56.22 |
1, 1, 1 | 7.43 | 88.60 | 1.43 | 55.98 | 30.00 | 5.32 | 44.81 |
-1, 1, 1 | 7.91 | 84.99 | 1.97 | 52.15 | 30.97 | 7.09 | 51.05 |
1, 1, -1 | 7.49 | 88.23 | 1.50 | 43.01 | 41.08 | 6.53 | 61.46 |
-1, 1, -1 | 8.26 | 84.74 | 1.99 | 41.78 | 39.57 | 8.23 | 67.97 |
1, -1, 1 | 7.62 | 87.52 | 1.64 | 53.67 | 30.90 | 6.31 | 45.83 |
-1, -1, 1 | 8.63 | 84.48 | 2.06 | 51.62 | 29.79 | 8.17 | 50.99 |
1, -1, -1 | 7.64 | 86.34 | 1.67 | 42.57 | 40.77 | 6.86 | 62.38 |
-1, -1, -1 | 8.84 | 84.34 | 2.05 | 41.46 | 38.13 | 8.97 | 68.41 |
0, 1, 0 | 7.69 | 86.06 | 1.78 | 46.47 | 37.62 | 6.90 | 56.50 |
0, -1, 0 | 7.88 | 85.03 | 1.94 | 45.66 | 37.42 | 7.35 | 57.06 |
0, 0, 1 | 7.72 | 85.57 | 1.85 | 51.93 | 32.40 | 6.71 | 47.93 |
0, 0, -1 | 7.83 | 85.11 | 1.91 | 43.32 | 39.82 | 6.99 | 63.78 |
1, 0, 0 | 7.51 | 87.91 | 1.56 | 46.19 | 39.36 | 6.06 | 53.88 |
-1, 0, 0 | 8.45 | 84.56 | 2.03 | 42.04 | 38.12 | 8.44 | 58.43 |
Relative standard deviation was never higher than 4%. |
Relative standard deviation was never higher than 4%.
HWS characterization shows high values due to the hemicellulosic fraction solubilization. In marine biomass, composed in this case by C. nodosa and different seaweed species (mainly brown algae), it compasses the alkali-soluble polyuronic compounds analogous to pectin, e.g., alginic acid, water-soluble reserve carbohydrates analogous to starch, e.g., laminarin and water-soluble ethereal sulphates like mucilages, e.g., fucoidan. HOL percentages were very significant representing an average of 47%. However, these values are lower than those found for some other marine plants without treatment. P. oceanica can have HOL content ranging 55–57% and CEL levels of 32% (Tarchoun et al. 2019). The Cymodoceaceae family have proportions of ~ 30% soluble carbohydrates and hemicellulose, ~ 50% cellulose and ~ 20% lignin and insoluble polysaccharide residue content (Macreadie et al. 2017). Normally cellulose content in marine plants is higher among seaweeds, besides some studies revealed high cellulose content in brown algae, where crude fibre is the highest (21.66–34.71%) compared to red and green algae (Ahmad et al. 2016). Bogolitsyn et al. studied the content of defatted protein-polysaccharide complex of different brown algae. Results showed 56. 51.30 and 46% of cellulose content for Laminaria digitata, Laminaria saccharina and Ascophyllum nodosum respectively (Bogolitsyn et al. 2020). Another study from Ge et al. determined a 30% of cellulose from the floating residue by-product from the alginate extraction process from Laminaria japonica (Ge et al. 2011). Moreover, it is significant that more than 84% of the cellulose is present as CEL (predominant in algae and bacteria) (Horii et al. 1987), which indicates that it can be used in specialized applications in pharmaceutical industry as diluent, disintegrant, glidant and a binder in direct compression.
LIG in algae is controversial, some authors suggest there is less lignin or no lignin at all (Yanagisawa et al. 2013), since lignified cell walls are considered an evolutionary feature of terrestrial plants from aquatic ancestors some 475 million years ago (Boyce et al. 2004), among other authors that confirmed the presence of lignin in seaweeds. Alzate-Gaviria et al., confirmed the presence of aromatic polyphenol complex or lignin-like compounds in macroalgae biomass composed by Sargassum natans and Sargassum fluitans with values of 25.40 and 29.50% respectively (Alzate-Gaviria et al. 2021). Martone et al., discovered the presence of secondary walls and lignin like those of vascular plants within cells of the intertidal red alga Calliarthron cheilosporioides (Martone et al. 2009). However, the LIG in marine plants and seaweeds are lower compared wood raw materials and agricultural wastes (Moral et al. 2017), which is an advantage when it comes to revaluating cellulosic materials.
Modelling and optimization
To improve the HOL and CEL recovery, a central composite experimental design was carried out to see the effect of selective lignin removal. All experimental data were fitted into equations 2 to 8 obtaining linear regression models.
$$\:ASH=0.23{X}_{T}^{2}+0.21{X}_{T}{X}_{t}+0.11{X}_{P}{X}_{T}-0.86{X}_{T}-0.36{X}_{t}-0.15{X}_{P}+7.78\:\:\:\left(2\right)$$
$$\:CEL=0.26{X}_{T}^{2}+0.13{X}_{T}{X}_{t}+0.89{X}_{T}+0.28{X}_{t}+0.14{X}_{P}+85.38\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)$$
$$\:EBE=-\:0.20{X}_{T}^{2}-0.10{X}_{T}{X}_{t}´-0.93{X}_{T}-0.28{X}_{t}-0.07{X}_{P}+1.87\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(4\right)$$
$$\:HOL=0.25{X}_{P}^{2}+0.21{X}_{T}+0.92{X}_{P}+45.25\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(5\right)$$
$$\:HWS=-0.30{X}_{P}^{2}-0.93{X}_{P}+37.87\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(6\right)$$
$$\:LIG=-0.85{X}_{T}-0.31{X}_{t}-0.355{X}_{P}+7.14\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(7\right)$$
$$\:YI=0.04{X}_{t}^{2}-0.32{X}_{T}-0.03{X}_{t}-0.94{X}_{P}+56.05\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(8\right)$$
ASH and EBE are negatively affected by temperature, time and H2O2 concentration due to harder conditions that help to remove remaining minerals and lipids respectively, allowing thus to recover more purified HOL and CEL. HWS was only negatively affected by the H2O2 concentration related to the removal of the hemicellulosic fraction by the oxidative reaction (Thi Thuy Van et al. 2022). LIG was inversely affected by temperature, time and H2O2 concentration which means a lower content in lignin like compounds by increasing the operational factors. This step helps to reduce the cellulose’s impurities, hemicellulose, and lignin like compounds by a depolymerization process. HOL recovery was positively affected by temperature and H2O2 concentration. The harshness conditions of H2O2 allowed to obtain HOL with a higher CEL yield because of the isolation of the cellulosic fraction and the removal of the lignin like compounds. CEL content shows proportional relation with temperature, time and H2O2 concentration, with higher dependency with temperature. In other terms, higher temperature, time and H2O2 concentration allow to recover a carbohydrate fraction that is richer in CEL. YI is negatively affected mainly by temperature and peroxide concentration, playing reaction time a minor role; by increasing the temperature and H2O2 charge, the YI decreases.
Table 3 summarizes the results of analysis of variance (ANOVA) just for F-Snedecor value, probability value and standard error of estimate (Std. error) of the predictor variables. Coefficients where accepted when p < 0.05. The highest F value was given by the temperature variable in ASH, CEL, EBE and LIG, being the highest (1484.33) in EBE. Linear temperature predictor was the most influencing parameter in these regressions. For HOL, HWS and YI the setting with major effect was the H2O2 concentration, being in HWS the only linear and quadratic predictor with impact.
Table 3
ANOVA of linear, quadratic and interaction of the independent variables.
Variance analysis | XT | Xt | XP | XT2 | Xt2 | XP2 | XTXt | XPXT |
ASH | F-Snedecor | 551.17 | 95.34 | 16.10 | 37.96 | --- | --- | 32.80 | 8.20 |
Probability | < 0.001 | < 0.001 | 0.004 | < 0.001 | --- | --- | < 0.001 | 0.02 |
Std. error | 0.02 | 0.02 | 0.02 | 0.03 | --- | --- | 0.02 | 0.02 |
CEL | F-Snedecor | 421.69 | 42.37 | 10.12 | 36.84 | --- | --- | 9.32 | --- |
Probability | < 0.001 | < 0.001 | 0.01 | < 0.001 | --- | --- | 0.01 | --- |
Std. error | 0.08 | 0.08 | 0.08 | 0.13 | --- | --- | 0.08 | --- |
| F-Snedecor | 1484.33 | 133.59 | 8.11 | 66.00 | --- | --- | 18.55 | --- |
EBE | Probability | < 0.001 | < 0.001 | 0.02 | < 0.001 | --- | --- | 0.01 | --- |
| Std. error | 0.01 | 0.01 | 0.01 | 0.01 | --- | --- | 0.01 | --- |
| F-Snedecor | 12.42 | --- | 229.80 | 16.87 | --- | --- | --- | --- |
HOL | Probability | 0.01 | --- | < 0.001 | 0.01 | --- | --- | --- | --- |
| Std. error | 0.35 | --- | 0.35 | 0.61 | --- | --- | --- | --- |
| F-Snedecor | --- | --- | 185.20 | --- | --- | 19.17 | --- | --- |
HWS | Probability | --- | --- | < 0.001 | --- | --- | < 0.001 | --- | --- |
| Std. error | --- | --- | 0.33 | --- | --- | 0.58 | --- | --- |
| F-Snedecor | 151.10 | 20.20 | 24.82 | --- | --- | --- | --- | --- |
LIG | Probability | < 0.001 | < 0.001 | < 0.001 | --- | --- | --- | --- | --- |
| Std. error | 0.08 | 0.08 | 0.08 | --- | --- | --- | --- | --- |
| F-Snedecor | 415.00 | 4.24 | 3555.41 | --- | 6.10 | --- | --- | --- |
YI | Probability | < 0.001 | 0.07 | < 0.001 | --- | 0.03 | --- | --- | --- |
| Std. error | 0.14 | 0.14 | 0.14 | --- | 0.24 | --- | --- | --- |
Table 4 collects the correlation coefficient (R), coefficient of determination (R2), adjusted coefficient of determination (R2-adjusted), standard error of estimate (Std. error), Constant Variance Test (Variance) and Normality Test Shapiro-Wilk (Normality).
Table 4
Statistical parameters values
Equation | R | R2 | R2-adjusted | Std. error | Variance | Normality |
ASH (2) | 0.99 | 0.99 | 0.98 | 0.06 | P = 0.81 (Passed) | P = 0.94 (Passed) |
CEL (3) | 0.99 | 0.98 | 0.97 | 0.24 | P = 0.93 (Passed) | P = 0.74 (Passed) |
EBE (4) | 0.99 | 0.99 | 0.99 | 0.02 | P = 0.36 (Passed) | P = 0.44 (Passed) |
HOL (5) | 0.98 | 0.96 | 0.95 | 1.11 | P = 0.79 (Passed) | P = 0.99 (Passed) |
HWS (6) | 0.97 | 0.95 | 0.94 | 1.05 | P = 0.56 (Passed) | P = 0.77 (Passed) |
LIG (7) | 0.97 | 0.95 | 0.93 | 0.25 | P = 0.84 (Passed) | P = 0.49 (Passed) |
YI (8) | 0.99 | 0.99 | 0.99 | 0.44 | P = 0.51 (Passed) | P = 0.32 (Passed) |
The resultant second-order polynomial models adequately represented the experimental data with the coefficient of determination ranging from 0.95 to 0.99, being high enough for fitting the model to the experimental data. All obtained equations passed the Constant Variance Test and the Normality Test Shapiro-Wilk, assuming that the linear regressions present heteroscedasticity and are well-modelled by a normal distribution.
3D response surface plots of the effect of interaction between the more predominant factors on dependant variables HOL, CEL and LIG are showed in Fig. 2.
The response surface plots showed no local minima or maxima around the central point for each of the responses and under the range of conditions tested because their individual optimization lies in the star points of the central composite design. Nonetheless, when taking several responses into account, the complexity notoriously increases. The optimal parameters for maximal LIG removal (5.32%) were 70ºC, 90 minutes and 6% w/w H2O2 concentration. They lead to higher HOL recovery (55.98%) with more proportion of CEL (88.60%), compared to milder conditions where LIG value was 8.97% and HOL recovery 41.46%, under 50 ºC, 30 minutes and 1% w/w H2O2 concentration.
The multiple programming method of More and Toraldo (Moré and Toraldo 1989) was used to identify the optimal values of the independent variables by providing the optimal operational conditions for the cellulose extraction. Table 5 shows the results for each optimal predicted dependent variable to obtain the values of the operational conditions for obtain the best response by the modelized equations.
Table 5
Operating conditions for obtain predicted optimal values of dependent variables
Content (%) | Predicted optimal values | Operating conditions | Normalized values |
T (ºC) | t (min) | [H2O2] (%) | XT | Xt | XP |
ASH | 7.46 | 70 | 90 | 6 | 1 | 1 | 1 |
CEL | 88.71 | 70 | 90 | 6 | 1 | 1 | 1 |
EBE | 1.45 | 70 | 90 | 6 | 1 | 1 | 1 |
HOL | 54.31 | 70 | 30 | 6 | 1 | -1 | 1 |
HWS | 30.81 | 70 | 30 | 6 | 1 | -1 | 1 |
LIG | 5.40 | 70 | 90 | 6 | 1 | 1 | 1 |
YI | 68.12 | 50 | 30 | 1 | -1 | -1 | -1 |
Figure 3 displays multiresponse contour plots for the three possible combinations of independent variables (plotted with Minitab® Analytical Software). White areas correspond to the combination of values that results in a product with a determinate composition. In this case, the elapsed independent variable is kept at its intermediate value. For ASH, CEL, EBE and LIG, the predicted optimal responses are obtained under the maximal operational conditions tested. The optimal cellulose recovery under these conditions will be of 88.71% and the higher delignification rate of 5.40%. By increasing the harshness of the conditions these rates could probably be augmented, but always in expenses of decreasing the YI, which is not always positive as this variable is optimized by milder operational conditions. In contrast, the HOL and HWS recovery are optimized with the higher temperature and H2O2 concentration and the lowest time setting.
Table 6 summarizes the deviations of the experimental values and the predicted optimal ones. As can we see, the experimental optimal values are well correlated with the predicted ones as Cook's Distance and Leverage values are acceptable. The responses to the considered optimal operational conditions are not influencing the obtained results by none of the selected dependent variables and cannot be considered outliers, giving therefore validity to the model. Table 7 represents the predicted optimized values of the dependant variables with different operational conditions under three different analysed cases: A, B and C.
Table 6
Experimental optimal values and their deviations from the predicted optimums
Content (%) | Experimental optimal values | Predicted optimal values | Std. residual | Cook's Distance | Leverage |
ASH | 7.43 | 7.46 | − 0.51 | 0.19 | 0.65 |
CEL | 88.60 | 88.71 | − 0.46 | 0.08 | 0.53 |
EBE | 1.43 | 1.45 | − 0.81 | 0.25 | 0.53 |
HOL | 55.98 | 54.31 | − 0.57 | 0.05 | 0.30 |
HWS | 29.79 | 30.81 | 0.08 | 0.01 | 0.20 |
LIG | 5.32 | 5.40 | − 0.32 | 0.02 | 0.37 |
YI | 68.41 | 68.12 | 0.65 | 0.09 | 0.40 |
Std. residual: standardized residual. Relative standard deviation was never higher than
Table 7
Optimization of responses under different tested operational conditions
| Content (%) | Predicted | Residual | Std. residual | Cook's Dist. | Leverage |
Case A | ASH | 7.46 | -0.03 | − 0.51 | 0.19 | 0.65 |
| CEL | 88.71 | -0.11 | − 0.46 | 0.08 | 0.53 |
| EBE | 1.45 | − 0.02 | − 0.81 | 0.25 | 0.53 |
| HOL | 54.31 | 1.67 | 1.51 | 0.35 | 0.30 |
| HWS | 30.81 | − 0.81 | − 0.77 | 0.06 | 0.20 |
| LIG | 5.40 | − 0.08 | − 0.32 | 0.02 | 0.37 |
| YI | 45.17 | − 0.36 | − 0.81 | 0.15 | 0.40 |
Case B | ASH | 7.59 | 0.03 | 0.57 | 0.25 | 0.65 |
| CEL | 87.21 | 0.31 | 1.29 | 0.65 | 0.53 |
| EBE | 1.64 | − 0.00 | − 0.04 | 0.00 | 0.53 |
| HOL | 54.31 | − 0.64 | − 0.57 | 0.05 | 0.30 |
| HWS | 30.81 | 0.09 | 0.08 | 0.00 | 0.20 |
| LIG | 6.12 | 0.19 | 0.76 | 0.13 | 0.37 |
| YI | 45.75 | 0.08 | 0.19 | 0.01 | 0.40 |
Case C | ASH | 8.86 | − 0.02 | − 0.27 | 0.06 | 0.65 |
| CEL | 84.15 | 0.19 | 0.80 | 0.25 | 0.53 |
| EBE | 2.08 | − 0.03 | − 1.44 | 0.81 | 0.53 |
| HOL | 41.19 | 0.27 | 0.24 | 0.01 | 0.30 |
| HWS | 39.87 | − 1.74 | − 1.66 | 0.29 | 0.20 |
| LIG | 8.88 | 0.09 | 0.36 | 0.03 | 0.37 |
| YI | 68.12 | 0.29 | 0.65 | 0.09 | 0.40 |
Std. residual: standardized residual. Relative standard deviation was never higher than 4%.
Case A) Optimization of ASH, CEL, EBE and LIG: Temperature 70 ºC; time 90 min; [H2O2] 6%. Under harsher experimental conditions the cellulose extraction gives the maximum response. The contour plot corresponding to these variables at maximum time can be seen in Fig. 4. The white region corresponds to combinations of H2O2 and temperature values that result in LIG values below 6%, hydrophobic extractives below 1.80%, ASH content below 7.40%, and CEL proportions above 87%. LIG, EBE, and minerals are minimized. This scenario is the most interesting when CEL extraction is the main purpose of the extraction process, being able to reach an optimum value of 88.71%. The high regenerated CEL content is comparable to chlorine delignified sugarcane bagasse (89.33%) cooking at 205 ºC (Phinichka and Kaenthong 2018) and Kraft softwood (86.60%) and hardwood (84.70%) (Iller et al. 2002). This material could have applications like this obtained from terrestrial feedstocks in traditional extraction processes. It can be applied for paper production and manufacture cellulose-derived products, such as cellulose ethers, cellulose esters and regenerated fibres or films (Sixta 2006).
Case B) Optimization of HOL and HWS: Temperature 70 ºC; time 30 min; [H2O2] 6%. The minimum time setting of 30 minutes allowed to get the optimal values for HOL and HWS recovery, giving the same values for the responses then in Case A but consuming less treatment time. The extraction process is characterized by the mechanical damage, thermal and chemical degradation that cellulosic fibres suffer under the reaction conditions. Milder operational parameters allow to reduce the fibres decomposition. In the HOL fraction the cellulose fibrils are longer, facilitating the alignment of the fibres and the determination of their orientation (Yang and Berglund 2021). Holocellulosic fibres have being tested as a wood fibre reinforcement material. These fibres present higher intrinsic strength, ductility and mechanical integrity being suitable for load-bearing applications (Forsberg et al. 2022).
Case C) Optimization of YI: Temperature 50 ºC; time 30 min; [H2O2] 1%. Milder operating conditions allowed to obtain higher YI but reduces the cellulose and HOL content. Besides a CEL content of 84.15% is more than suitable for paper industry pulps, the HOL content of 41.19% is too low for compete with woody plants and agricultural residues. The HWS fraction of the slurry was higher than the obtained under the above-mentioned conditions with a content of 39.87%. This fraction constituted mainly by soluble carbohydrates that could be refined for obtaining added-value compounds present in brown seaweeds like alginates, laminarins and fucoidans that can be used for pharmaceutical and nutraceutical purposes or for production of biofertilizer, food or bioethanol (Matos et al. 2021).
Depending on the expected revalorization outcome, the modelling of the extraction process by multiple linear regression analysis and optimization methods allow to obtain materials with different composition and characteristics. The tidal waste chemical composition can be compared with other feedstocks employed as cellulosic material source, like agricultural residues and woody sources, obtained under similar extraction conditions (Table 8). This comparison under similar conditions can give an idea of the revalorization possibilities for the tidal residue.
Table 8
Chemical composition for different raw materials under NaOH -AQ (S1)-and AHP (S2) stages
ASH (%) | CEL (%) | EBE (%) | HOL (%) | HWS (%) | LIG (%) | YI (%) | Reference |
Tidal waste S1: [NaOH] = 4%, [AQ] = 0.50%, L/S = 20, T = 130 ºC, t = 120 min. S2: [H2O2] = 6%, L/S = 10, T = 70 ºC, t = 30 min. | This study |
7.59 | 87.21 | 1.64 | 54.31 | 30.81 | 6.12 | 45.75 | |
Arenga pinnata S1: [NaOH] = 10%, [AQ] = 0%, L/S = 10, T = 150 ºC, t = 120 min. S2: [H2O2] = 5%, L/S = 10, T = 60 ºC, t = 90 min. | Fitriana et al. 2020 |
0.18 | --- | --- | 94.48 | --- | 5.34 | --- | |
Bamboo chips S1: [NaOH] = 8%, [AQ] = 0%, L/S = 10, T = 100 ºC, t = 180 min. S2: [H2O2] = 6%, L/S = 10, T = 75 ºC, t = 180 min. | Yuan et al. 2018 |
--- | --- | --- | 71.50 | --- | 13.80 | 58.30 | |
Brewer’s spent grain S1: [NaOH] = 2%, [AQ] = 0%, L/S = 20, T = 120 ºC, t = 90 min. S2: [H2O2] = 5%, L/S = 20, T = 70 ºC, t = 40 min. | Mussatto et al. 2008 |
1.40 | --- | --- | 74.50 | --- | 12.60 | 78.80 | |
Grass waste S1: [NaOH] = 1%, [AQ] = 0%, L/S = 4, T = 55 ºC, t = 360 min. S2: [H2O2] = 4%, L/S = 4, T = 55 ºC, t = 360 min. | Macreadie et al. 2017 |
--- | --- | --- | 71.60 | --- | 8.20 | 54.30 | |
Sago Frond S1: [NaOH] = 10%, [AQ] = 0%, L/S = 20, T = 100 ºC, t = 120 min. S2: [H2O2] = 5.43%, L/S = 2, T = 95 ºC, t = 60 min. | Arnata et al. 2019 |
--- | --- | --- | 62.86 | --- | 9.16 | 38.50 | |
Sugarcane Straw S1: [NaOH] = 20.60%, [AQ] = 0.15%, L/S = 12, T = 160 ºC, t = 210 min.+ S2: [H2O2] = 9.40%, L/S = 20, T = 60 ºC, t = 60 min. | Costa el al. 2013 |
--- | --- | --- | 90.00 | --- | 0.50 | 38.00 | |
Wheat straw S1: [NaOH] = 1.25%, [AQ] = 0%, L/S = 10, T = 30 ºC, t = 120 min. S2: [H2O2] = 6%, L/S = 10, T = 50 ºC, t = 180 min. | Yuan et al. 2018 |
--- | --- | --- | 78.80 | --- | 18.10 | 77.60 | |
Relative standard deviation was never higher than 4%.
Despite the lack of information, the YI value could be compared, being the tidal waste YI (45.75%) between the range of terrestrial cellulosic materials (38.00-78.80%). The other comparable response with references is HOL, which showed a lower value (54.31%) compared with terrestrial sources (62.86–94.48%). Due to the characterization method here employed, it can be assumed that the HOL content of the tidal waste can be considered as mostly cellulose as the main hemicellulosic portion is almost comprised in the HWS characterization. This is since seaweeds and marine species hemicellulosic fraction is alkali-soluble and water-soluble. On the other hand, LIG was one of the lowest (6.19%) compared to the feedstocks after the AHP process (5.34–18.10%), which is beneficial as the recalcitrant nature of lingin increases the chemical consumption and the processing conditions (Hu et al. 2018). Moreover, the employed time in the H2O2 stage of this study (30 min) was the lowest compared to terrestrial feedstocks (40–360 min).