Application of Lignin for Slope Bioengineering: Effect on Soil Improvement and Plant Growth

Abstract

This study aims to establish whether lignin-treated soils could result in greater soil strength and stimulate seed germination and growth, which can be essential for slope bioengineering. Three different soil types with a range of plasticity were treated with lignin solutions of 1% and 3%. The changes in soil strength and seed growth were observed for 40 days to simulate the long-term field performance. Two methods to treat the soils were employed: Method 1 involved mixing lignin solutions with the whole soil sample, while Method 2 involved spraying the lignin solutions on the already-prepared soil sample. The results indicated that the lignin concentration and the soil treatment method could affect soil strength, whereas soils treated with 3% lignin solution using Method 1 consistently produced greater soil strength values. The lignin-treated soils were able to retain more moisture at the end of the experiment than the untreated soils. Both lignin-treated and untreated soils produced similar results on seed germination and growth, suggesting that lignin does not have a negative effect on slope bioengineering.

1. Introduction

Slope erosion has long been an environmental and geotechnical concern in many countries, resulting in economic loss. Different engineering solutions, mostly involving cement and geosynthetics, have been successfully used in practice to tackle this issue. However, recent research has shown that alternative environmentally friendly products like biopolymers can also be employed for soil stabilization [1,2,3,4,5,6,7,8]. For instance, lignin, a complex and heterogeneous biopolymer that is a component of plant cell walls, has lately drawn the attention of the scientific community due to its availability and relatively low cost. Lignin is a by-product of sugar cane and paper mill production, which is considered industrial waste. Several studies [8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29] have been performed on soil treated with lignin, predominantly focusing on soil strength improvement in the laboratory environment, with limited research on its long-term field performance. In addition, although a few studies have been performed on biopolymers [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51], only limited research on the effect of lignin on seed germination and growth has been conducted to date. Vegetation can decrease soil erosion and stabilize slopes by increasing the strength of soil mass by vegetation roots and reducing water runoff on the slope surface.

This study aims to improve our understanding of the use of lignin in current engineering and environmental practices. The objectives of this study were to establish (a) how lignin solutions could affect the strength of different soils, (b) what method of lignin application could produce superior outcomes and (c) how lignin could affect germination and growth of vegetation. The results of this study will enhance understanding of soil improvement with lignin and assist engineers and practitioners with selecting alternative environmentally friendly solutions to erosion issues.

2. Literature Review

Effect of lignin on soil properties. Several studies have been conducted to understand the effect of lignin on the engineering properties of soil, including its strength.

Table 1. Summary of studies on the effect of lignin on soil properties.

Soil Type	Lignin Type	Additive Dosage (%)	Curing Period	Test Method	Key Findings	Reference
Aggregate mixed	Lignosulfonate	-	7–28 days	UCS, durability	Lignin enhances UCS and SFS. The durability of the soil improves by lignin under wet–dry and freeze–thaw cycles.	Bolander [14]
Clays	Lignosulfonate	0.5, 0.6, 1, 1.5	0–28 days	UCT, EC	Strength and stiffness increase with lignin content, while increasing cement content decreases them. EC of lignin-treated soil decreases with lignin. Lignin reacts with clay minerals per the electrostatic reaction.	Indraratna et al. [15]
Sandy silt	Lignosulfonate	0.5, 1, 2, 3, 4	7–28 days	TT	UCS and E increase significantly with lignin at 2%. Post-peak ductility of lignin-treated soil remained unchanged compared to untreated soil. Lignin-treated soil shows a significant dilation compared to untreated soil.	Chen et al. [16]
Silty soil	Lignin, quick lime	2, 5, 8, 12, 15	0–60 days	UCT, CBR, Vs	Lignin-treated silt soil achieved optimum performance at 12% lignin content and a curing age of 28 days.	Zhang et al. [17,18,19]
Silt soil	Lignin, lime, cement, fly ash	7, 10, 12, 14, 16	1–28 days	UCT	UCS of lignin-treated silt increased at a peak of 12% lignin and 28 days curing age.	Kong et al. [20]
Silty soil	Sulfur-free lignin	3, 7, 10, 12, 15	1–60 days	UCT	UCS increased five times at 10% lignin content compared to untreated soil after 60 days.	Liu et al. [21,22]
Black cotton soil	Sodium lignosulfonate	1, 3, 6, 9, 12	3, 7, 28 days	UCT, DST	UCS increased by about 43% at 9% lignin dosage at 28 days curing age. c increased at 12% lignin content.	Singh et al. [23]
Dispersive soil	Calcium-lignosulfonate	0.5, 1, 2, 3, 4	28 days	UCT	UCS increased by about 50% at 0.5% lignin content at a curing age of 28 days.	Ji et al. [24]
Clay mixed granite sand	Calcium-lignosulfonate	0.5, 1, 1.5, 2	0, 7, 14, 28, 90 days	UCT, HT	The maximum UCS achieved at 0.5% lignin content for the clay mixtures. K decreases with lignin content.	Amulya et al. [25]
Clay	Lignin-coir fibers, lime	0.5, 1, 1.5	–	UCT, CBR, VST	CBR increase, at 1.5%. UCS and shear strength increase at 1% 1% lignin-coir fiber and 5% lime mixed was found to be the optimum.	Boobalan and Sivakami, [26]
Silty soil	Calcium lignosulfonate	0.5, 1.0, 1.5	28 days	UCT, DST	Lignin dosage of 1% was the optimum for shear strength. Lignin content of 0.5% was the optimum for UCS.	Du et al. [27]
Clay soil	Calcium lignosulfonate, granite sand	0.25, 0.5, 1, 1.5; 30, 40, 50	7, 14 days	DST	c and φ achieved optimum value at 30% granite sand and 0.5% lignin content.	Varsha et al. [28]
Silty soil	Lignosulfonate	1, 3	1–35 days	TT, UCT	c, φ, and UCS achieved optimum value at 3% lignin content.	Bagheri et al. [8]
Clay soil	Sodium lignosulfonate	0.5, 1.0, 1.5	7, 28, 90	UCT	Lignin additive value of 1% is considered the optimum for UCS and shear wave velocity.	Vakili et al. [29]

Note: UCT—unconfined compressive strength; EC—electrical conductivity; TT—triaxial test; c—cohesion; φ—friction angle; VST—vane shear test; K—hydraulic conductivity; DST—direct shear test; HT—hydraulic conductivity test.

summarizes the insights from these studies with a focus on the soil type, lignin concentration and the type of testing. A few well-documented studies are briefly discussed below. Tingle and Santoni [9] used lignin to stabilize clay soil and reported that the unconfined compressive strength (UCS) of lignin-treated clay improved significantly at a 5% lignin concentration. Al Mahamud et al. [10] used lignin sulfonate and cement (0.6%, 1%, 2%) to enhance the strength properties of clays. The investigators noted that the UCS increased with lignin content while increasing cement content more than a certain point decreased UCS. Bai et al. [11] showed that 3% of lignin increased the UCS of loess (silty soil) by about 180%. Loranger et al. [12] reported that the addition of lignin (1–2%) increased the mechanical strength of quarry sand by more than 60%. Vaina et al. [13] found that the California bearing ratio (CBR) of lignin-treated clay significantly increased after 7 days of curing age.

In summary, the previous studies primarily dealt with the engineering properties of plastic clay or silt, while limited research was conducted on cohesionless materials such as sand and gravel. The majority of studies employed a series of UCS tests in which cylindrical specimens of soil were treated with lignin to obtain UCS. In these studies, the soil was thoroughly mixed with lignin solutions with a relatively low concentration of 1–5% and cured at a different time (mostly 7–10 days).

Effect of biopolymers on vegetation growth. Vegetation is an essential part of the bioengineering of slopes as it creates a mesh structure with interconnected roots, forming a root-reinforced soil mass. Several studies, summarized in

Table 2. Summary of studies on biopolymer and vegetation growth.

Soil Type	Biopolymers	Content (%)	Method	Vegetation Type	Curing Days	Outcomes	Reference
Sand	Lignin-(guaiac, syringyl, p-hydroxyl phenyl)	2	Mixing—spraying	Agriophyllum squarrosum, Artemisia desertorum Spreng, etc.	120–150	The lignin biopolymer and the species used could form a community within 2 to 3 years and stabilize the desert dune significantly.	Hanjie et al. [34]
Red–yellow sand	Xanthan gum, β-glucan	0.5	Mixing	Oats (600 seeds)	7–21	Xanthan gum and β-glucan biopolymers stimulated seed germination and growth in natural and cultured soil.	Chang et al. [39]
Granular soil	IPCs, HPAN, PDADMAC, KNO3	1–2 wt.%	Mixing	Sudan grass	1095–1460	IPCs offer a considerable soil binder and enhance grass growth compared to the other polymers.	Zezin et al. [35]
Clayey soil	Xanthan gum, guar gum, agar gum, beta-glucan	0.25, 0.5, 0.75, 1	Mixing	Oats (160 seeds)	2–14	Xanthan gum efficiently promotes vegetation growth and increases the seed germination ratio by about 300% at 0.5% compared to other biopolymers.	Ni et al. [40]
Silt	Xanthan gum, guar gum, agar gum	0.5	Mixing	Ryegrass	7, 14, 21, 28	Xanthan gum-treated silt has the highest growth and germination rate compared to guar- and agar-treated silt.	Wang et al. [44]
Sand	Xanthan gum-starch	0.5	Spraying	Turfgrasses	16	Xanthan gum enhanced water retention, soil cohesion, and vegetation growth to improve erosion resistance.	Tran et al. [33]
Sand	Polyurethane	0–20	Mixing	Ryegrasses (60 g of seeds)	10–60	Polyurethane polymer did not show effective promotion of vegetation growth compared to untreated sand, especially at concentrations above 10%.	Liu et al. [46]
Sand	Xanthan gum, cationamyl, potassium nitrate, carboxy methyl cellulose	0.5–1	Mixing	Festuca, Poa pratensis, Lolium perenne and Trifolium repens seeds	1–27	Biopolymers are capable of improving the overall vegetation growth.	Nikolovska et al. [37]
Weathered granite soil	Xanthan gum, starch, β-glucan	0.45–0.5	Mixing—spray	Seeds	30	Xanthan gum, starch, and β-glucan compounds were implemented using the wet-spraying method to strengthen the structure and promote vegetation growth on levee slopes.	Seo et al. [47]
Sand	Hydrophilic polysaccharide biopolymer (HPB)	0.05–1	Spraying	Ryegrass and Bermuda grass	12	HPB concentration of less than 5% promotes vegetation growth.	Che et al. [48]
Clay soil	Xanthan gum	0.25–1	Mixing	Oats (160 seeds)	1–14	Xanthan gum promotes vegetation growth at 0.25% to 0.5% dosage; above 0.5% may impede vegetation growth.	Ni et al. [49]
Desert sand	MICP	0.1–0.5 M	Spraying	Heracleum persicum	7–30	MICP biopolymer promotes the germination of Heracleum persicum.	Naeimi et al. [36]
Loess	Xanthan gum	0–1	Mixing	Oats (160 seeds)	14	Xanthan gum yields higher germination from 0.25% to 1% dosage and high root content at 0.25% to 0.75%.	Ni et al. [50]
Clay	Xanthan gum	0–4	Mixing	LP seeds	60	Xanthan gum promotes LP growth, and increasing Xanthan dosage increases the growth rate.	Wan et al. [51]

Note: IPCs—inter-polyelectrolyte complexes; HPAN—Hydrolyzed Polyacrylonitrile; PDADMAC—PolyDiallyldimethylammonium Chloride; KNO3—potassium nitrate; MICP—microbially induced calcium carbonate precipitation.

, have been conducted on vegetation growth in relation to slope stability [30,31,32], erosion control [33], dusting [30,34,35], microbially induced calcium carbonate precipitation [36] and vegetation [34,37,38,39]. Chang et al. [40] explored xanthan and β-glucan gums with red–yellow soil and reported that biopolymer-treated soil promoted the soil environment for vegetation growth. Ni et al. [41] used xanthan, guar, agar, and beta-glucan gums in laboratory experiments with clayey soil and reported that these biopolymers promoted vegetation growth and improved seed germination. Using field studies, Cho and Chang [42,43] revealed that biopolymers such as xanthan gum, guar gum, and agar gum were sustainable materials for promoting surface vegetation growth. Ko et al. [44] observed that biopolymer-treated river sand slopes with grass increased slope stability and erosion resistance.

A recent review study by Savy and Cozzolino [45] summarized advances in the use of lignin in agriculture to stimulate plant development; however, only limited research on the effect of lignin on vegetation growth in relation to slope bioengineering has been conducted to date. A recent study [34] showed that lignin could play a significant role in soil–plant interactions and improve vegetation growth within 2 to 3 years, helping to stabilize desert dunes. However, due to the limited research, the effect of lignin on seed germination and vegetation growth remains unclear and needs further investigation.

3. Materials and Methods

This study included a series of experiments on three different soil types treated with different solutions of lignin. The change in the soil strength over 40 days was estimated by conducting pocket penetrometer and vane shear tests. Three types of seeds were used to estimate the effect of lignin on seed germination and growth. The following section details the materials used and the experimental procedure.

3.1. Materials Used

Three different soil types were used: Soil 1 (black clay), Soil 2 (brown silt), and Soil 3 (sand). The rationale for this selection was to investigate the effect of lignin on the properties of high-plasticity (Soil 1), low-plasticity (Soil 2), and non-plastic (Soil 3) soils. A series of standardized laboratory tests were performed to estimate soil plasticity [52], maximum dry density and optimum water content [53], and specific gravity tests [54]. Additionally, X-ray tests were conducted to estimate the soil mineral composition. The following data were obtained:

Soil 1: High-plasticity black clay with a liquid limit of 79.2% and a plasticity index of 46.0%, which is classified as CH, according to the Unified Soil Classification System (USCS). The optimum moisture content (OMC) was 27.0%, with a maximum dry density of 1.35 g/cm³. The specific gravity was 2.71. The results of the X-ray diffraction test revealed the following dominant clay minerals: smectite, kaolinite, and illite.

Soil 2: Low-plasticity brown silt (USCS classification is ML) had a liquid limit of 38.0% and a plasticity index of 11.1%. The specific gravity was 2.77, the maximum dry density was 1.72 g/cm^3, and the optimum moisture content was 21.7%. The mineral composition included quartz, kaolinite and calcite.

Soil 3: Well-graded non-plastic sand (USCS classification was SW), which had the maximum dry density of 1.85 g/cm³ with an optimum water content of 13.0%. It predominantly consisted of quartz.

Lignin. The ammonium lignosulfonate lignin was commercially acquired from Dustex, Australia. The properties of lignin were as follows: calcium lignosulphonate lignin was a brown viscose liquid with a pH (10% solution) of 5.4 ± 3.0, dry matter of 55.0 ± 1.0%, and a density of 1285 kg/m³. The lignin was a mixture of water (51%) and calcium lignosulfonate (49%). The lignin solutions used to treat the soils were prepared by mixing lignin with distilled water. Two solutions of lignin were prepared; that is, 1% and 3% of the lignin mass to the total mass of the dry soil. It is noted that solutions of lignin with greater concentrations (>3%) were too viscous, and it was challenging to mix such solutions with soil.

Seeds. Commercially available Couch (Cynodon dactylon), Rhodes grass (Chloris gayana), and Forest blue grass (Bothriochloa bladhii) [55] were utilized in this study as these native seeds have been widely used in engineering practices in Queensland, Australia.

3.2. Experimental Program

Laboratory work consisted of soil examination, specimen preparations, a series of pocket penetrometers and hand vane shear tests to measure the change in soil strength over time, and observation and monitoring of seed growth in soil with and without lignin. The pocket penetrometer tests were conducted using a Geo-Pocket penetrometer and a 6.4 mm plunger with a capacity of 0–6 kg/cm². The standardized vane shear tests were conducted according to ASTM D2573-08, the Standard Test Method for Field Vane Shear Tests in Cohesive Soil [56].

3.2.1. Soil Specimen Preparations

Two methods were used to treat the soil with lignin: Method 1 involved mixing soil with lignin solutions, and Method 2 was about spraying lignin solution on the already-prepared soil samples. The rationale for using two methods was as follows: Method 1 was used in previous laboratory studies to prepare specimens for unconfined compression and triaxial tests [3,8]. In this method, the whole soil mass was mixed with lignin solutions to produce small-sized specimens. This method proved effective for a relatively small amount of soil involved, but it might be rather expensive in practice. Method 2 appeared more cost-effective in real engineering practice [57], but in this case, the lignin solution was only applied on the surface of the soil mass.

Method 1. The lignin was mixed with distilled water to produce lignin solutions of either 1% or 3%. The lignin solutions were then mixed with the oven-dried soil to achieve the desired value of the initial water content, which was equal to the optimum water content of each soil. The solution was manually mixed with the soil for 30 min. The treated soil was sealed in a plastic bag and allowed to rest in a controlled temperature room for 24 h to ensure an even moisture distribution in the whole sample. The soil was then placed into an aluminum foil tray with a length of 31.5 cm, width of 26 cm, and height of 5 cm. Each soil was compacted in two layers in the tray to achieve the maximum dry density. However, it was challenging to achieve such high values of dry density due to technical issues during the compaction process. The following values of dry density were achieved: 1.30 g/cm³ for Soil 1, 1.63 g/cm³ for Soil 2, and 1.77 g/cm³ for Soil 3, which were within the 95% range of the dry density.

Method 2. The oven-dried soil was first mixed with distilled water to prepare soil specimens with the same initial water content as in Method 1. The moist soil was placed in a sealed plastic bag and allowed to rest in a controlled-temperature room for 24 h to ensure an even moisture distribution. The soil was placed in an aluminum tray of the same size as in Method 1 and compacted in two layers to achieve the same value of dry density as in Method 1. The lignin solutions of the same concentration (1% and 3%) as in Method 1 were sprayed (700 mL) on top of the soil surface.

For each method, the soil sample in the tray was divided into three equal parts: the left part was for Couch seeds, the middle part was for Rhodes grass seeds, and the right part was for Forest blue grass seeds. The seeds were spread evenly in their designated parts, and the following seed mass was used following the government’s recommendations for garden areas: 0.3 g for Coach, 0.7 g for Rhodes grass, and 0.7 g for Forest bluegrass. The seeds were covered in a thin layer of soil and watered. To help the seeds germinate and grow, 100 mL of water was sprayed on the soil surface twice per week.

3.2.2. Soil Testing

Pocket penetrometer (PP) tests were regularly conducted to estimate the change in soil strength over 40 days. At least five tests were performed in different locations at each measurement, and an average PP value (in kPa) was obtained, reported, and analyzed. To estimate the unconfined compressive strength (UCS) of the studied soils, the empirical correlation suggested by Burt [58] was used, where UCS = 0.8 × PP. Note that this test is typically conducted on cohesive soil, and it is challenging to perform this test on cohesionless material like sand or gravel. For this reason, the PP data could only be obtained for Soil 1 and Soil 2.

To estimate the shear strength of soil with and without lignin, a standard vane shear test was conducted at the end of each experiment (40 days from the beginning). After the vane shear test, the soil specimen in each experiment was cut through for visual observations of the depth of the lignin treatment. In addition, at the end of each experiment, soil samples were collected for water content tests to determine the amount of moisture that remained in the soil after 40 days.

4. Results and Discussion

4.1. Compressive and Shear Strength Characteristics

Pocket penetrometer tests. Figure 1 and Figure 2 show the results from a series of PP tests conducted over 40 days on each soil treated with different lignin concentrations. These results are compared with the data obtained for the untreated soil (only water) to estimate the positive contribution of lignin treatment.

As can be seen in Figure 1, the strength of Soil 1 treated with lignin increased over time; however, so did the strength of the untreated soil. The latter can be attributed to the soil surface drying, which was observed during testing. However, compared to the untreated soil, the soil mixed with lignin solutions consistently produced greater values of soil strength. The difference in strength (almost twofold) was the most pronounced when Method 1 was used. According to Bagheri et al. [3], an increase in the strength of lignin-treated soil can occur because lignin fills the pore space between soil particles and adds a biopolymer coating to the particles. A similar tendency was observed for Soil 2 (Figure 2), where the soil specimen mixed with 3% lignin solution exhibited a much greater value (1.5–2 times compared to the untreated soils) of strength over time.

The comparison between the lignin treatment methods indicates that the mixing of soil with lignin (Method 1) was associated with greater increases in soil strength. It can also be observed from Figure 1 and Figure 2 that, for both Soils 1 and 2, the strength of soil treated with lignin solutions by Method 2 was similar to the strength of untreated soil.

The data given in Figure 1 and Figure 2 suggest that the addition of lignin could increase the strength of both high (Soil 1) and low (Soil 2) plasticity soil, especially when Method 1 was used. Interestingly, for both soils, the noticeable increase in the strength of lignin-treated soils began to occur after 7–10 days from the beginning of the experiment. This finding correlated with the laboratory results of previous studies that indicated that a curing time of 7–10 days was sufficient to reach a high value of strength.

Although an increase in soil strength over time can be observed in Figure 1 and Figure 2, the obtained points tend to fluctuate. This data variation can be attributed to the following reasons:

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For each measurement, at least five pocket penetrometer tests were conducted, and the average value was used to estimate the soil surface strength. The PP measurements were performed in different parts of the soil samples. As the soil mass was not homogenous (due to the soil nature and experimental preparations), different values were sometimes obtained, depending on the point selected for the measurement.

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For each experiment, the soil surface was sprayed with 100 mL of water twice per week to stimulate seed germination and growth, and changes in moisture on the soil surface contributed to the soil inhomogeneity.

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Over time, the soil samples had a natural tendency to dry on the surface, initiating surface cracks. This process contributed to the soil inhomogeneity as well, and impacted the measured soil strength.

Vane shear tests. The results of vane shear tests are summarized in Figure 3. Despite the soil type, similar results were obtained; that is, the specimens treated with lignin solutions produced greater strength than the untreated specimens. Interestingly, the vane shear strength of the black and brown soil specimens mixed with 1% lignin solution exhibited a greater strength than the specimens treated with the 3% lignin solution. Also, when the lignin solution was sprayed on the surface of all three soil specimens, the vane shear strength of the specimen with 3% lignin was always slightly greater than the strength of the specimens treated with 1% lignin solution. The obtained data suggest that treatment of all three soils with the lignin solution increased the vane shear strength of the soil. Also, similar to the pocket penetrometer test data, Method 1 produced the soil specimens with greater vane shear strength compared to Method 2.

The vane shear test is typically conducted on soft, cohesive soils like Soils 1 and 2. However, it was also possible to obtain a vane shear strength value for Soil 3 because, at the end of the long-term experiment, the sand developed apparent cohesion due to soil suction [59]. In addition, the treatment of the sand with lignin solution significantly increased its vane shear strength (more than six times the untreated soil strength, Figure 3c), suggesting that lignin helped to develop adhesive bonding between the sand grains, making it stronger. These results appeared promising for the possible bioengineering treatment of slopes consisting of sand.

Effect of lignin on soil strength. The experimental data indicated that adding lignin to each tested soil generally led to an increase in soil strength. The obtained results are replotted in Figure 4 as the percentage of soil strength changes over time in comparison to the strength of the untreated soil specimens. For the lignin-treated Soil 1, the maximum increase in strength occurred after the first ten days, whereas Method 1 was associated with the greatest (more than 100%) increase in strength. Interestingly, the percentage of strength increase gradually diminished over time but remained positive (about 40%) after 40 days. However, for the soil specimens prepared by Method 2 (spray), the change in strength, compared to the untreated soil, became negligible after 30 days.

Unlike Soil 1, Soil 2 treated with lignin showed no increase in strength after 10 days, compared to the untreated specimens. However, the strength gradually increased over time, especially for Method 1, when the strength increased by 60% and 120% for the specimens mixed with 1% and 3% lignin, respectively. For the specimens sprayed with 1% lignin, no significant change in the strength was observed after 40 days.

The results from the vane shear tests (Figure 5) indicated that for each soil and lignin concentration, the vane shear strength of the treated soil exceeded the strength of the untreated soil. Method 1 significantly improved the soil strength with at least a 100% increase for each soil type compared to the untreated soil. Method 2 also produced soil samples with a greater strength than the untreated soil, but the increase was smaller, generally in the range of 50–100%.

4.2. Visual Observations and Water Content

When lignin-treated soils underwent natural drying, a firm surface was formed. This dehydrated soil was strong and stiff enough to provide additional strength. This process was the most pronounced for Soil 3 mixed with 1% (Figure 6b) and 3% (Figure 6a) lignin solution. Interestingly, the lignin-treated soils managed to retain more moisture compared to the untreated soils by the end of the experiment.

The data on water content measured for each soil after 40 days are given in Figure 7. The increased water content was observed for Soil 1 treated with lignin solutions. Soil 3, treated with lignin, was also able to hold more water than the untreated sample. The water retention capacity of the lignin-treated soils can be a compelling factor for vegetation growth in places with limited precipitation. Previous studies have shown that biopolymers, including lignin, can lead to the formation of electrical charges around the soil particles that have a stronger bond with water molecules [39,60,61].

4.3. Seed Germination and Growth

Vegetation is an essential factor in increasing soil resistance to erosion. The vegetation root system can reinforce soil mass, preventing soil particle detachment. Fast vegetation growth contributes to effective soil stability treatment; however, the vegetation can fail due to insufficient precipitation (water access). In this study, seed germination and growth were monitored over 40 days, and the following observations were made: (1) The seed growth occurred for all types of soil and all concentrations of lignin (Figure 8 and Figure 9). (2) Rhodes grass growth was much better than the growth of the other two types of seeds. This was the case for all types of soil and lignin concentrations. (3) Better growth was observed for Soil 1 compared to the other two types of soil. (4) Less growth was observed for Soil 2 as its surface underwent significant drying. (5) The spray method was associated with better vegetation growth for all three soils.

To stimulate seed germination and growth, 100 mL of water was sprayed on the top of each soil twice per week. The rationale was to establish whether the seeds would germinate in lignin-treated soil and how it would be different from untreated soil under the same conditions. It is noted that daily watering and precipitation-controlled conditions would provide a better environment for seed growth [39], while less watering could result in fewer germinations [34] compared to the present study. The results indicated that considering the amount of watering in this study, the lignin-treated soils provided a suitable environment for seed germination and growth, comparable with the untreated soil. This study showed similar rates of seed germination and growth for both lignin-treated and untreated soils.

5. Conclusions

In this study, a laboratory investigation of different soil types treated with 1% and 3% lignin solutions was conducted to understand the effect of lignin on soil strength and seed growth. Based on the obtained data, the following conclusions can be drawn:

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The application of lignin biopolymer increased the strength of all three tested soil types. Both pocket penetrometer and vane shear test results indicated that the soil treated with lignin produced greater strength, especially when a 3% lignin solution was used. For high-plasticity Soil 1, the maximum increase in the strength, compared to the untreated control soil sample, occurred after the first 10 days, while for low-plasticity Soil 2, the maximum increase in the strength occurred later, within 20–30 days.

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The increase in soil strength was greater when lignin solutions were mixed with soil (Method 1) compared to the other method (Method 2) when lignin solutions were sprayed on the surface of the soil mass.

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The lignin-treated soil could retain more water over the long term compared to the untreated soil. This was more pronounced for very plastic soil (Soil 1) and non-plastic sand (Soil 3) than for Soil 2. This extra moisture could contribute to better vegetation growth, leading to greater resistance to soil erosion.

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Both lignin-treated and untreated soils produced similar results on seed germination and growth, suggesting that lignin does not have a negative effect on vegetation. The outcomes of this study indicate that the addition of lignin to soil can improve soil strength without negative effects on the environment and vegetation growth, which is important for alternative slope bioengineering methods.