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Article

Effects of Humus and Solidification Agents on the Solidification/Stabilization Process of Organic-Rich River Sludge: Characteristics of the Stabilized Sludge

by
Yuqi Zhu
1,2,3,†,
Fuyuan Ran
1,2,3,†,
Sihong Liu
4,
Liujiang Wang
4,* and
Chunzhen Fan
1,2,3,*
1
National and Local Joint Engineering Research Center of Ecological Treatment Technology for Urban Water Pollution, Wenzhou University, Wenzhou 325000, China
2
Zhejiang Provincial Key Lab for Water Environment and Marine Biological Resources Protection, Wenzhou University, Wenzhou 325000, China
3
College of Life and Environmental Science, Wenzhou University, Wenzhou 325000, China
4
College of Water Conservancy and Hydropower Engineering, Hohai University, Nanjing 210098, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Water 2025, 17(8), 1153; https://doi.org/10.3390/w17081153
Submission received: 7 March 2025 / Revised: 9 April 2025 / Accepted: 10 April 2025 / Published: 12 April 2025
(This article belongs to the Special Issue Sustainable Water Treatment Systems: Green Infrastructure and Bioremediation)
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Abstract

:
River sludge usually contains a high content of organic matter, leading to its low strength or difficult solidification in its solidification/stabilization (S/S) treatment projects. This study selected river sludge with medium and high content of organic matter for the S/S treatment using modified curing agent (GCP) and cement (P.O). Effects of humus and curing agent on the S/S process of river sludge were investigated via analyzing physical properties, changes in organic matter, microstructure, and mineral compositions of the solidified sludge. The results showed that the increase rate of compressive strength of the solidified sludge was influenced by the content of organic matter and composition of the curing agent. The presence of humus inhibited the hydration reaction and reduced the increase rate of compressive strength of solidified sludge. Slag and phosphogypsum in GCP promoted the hydration reaction, significantly enhancing the compressive strength of the solidified sludge to 2242.24 KPa. The water content of the solidified sludge was influenced by the environmental conditions and curing agent, which could reflect the level of hydration reaction in the solidified sludge. The pH of the solidified sludge was directly affected by the humus in the sludge, with a decreasing trend during the S/S process. Decomposition of the humus in the sludge released H+, which reacted with OH− produced by the hydration reaction via neutralization reaction. The pH of the solidified sludge was lowered, and the hydration reaction was inhibited, hindering the decrease in the water content of the solidified sludge. Therefore, the hydration reaction has an antagonistic effect on the decomposition of the humus. Microstructure analysis (SEM) confirmed that GCP could effectively solidify the organic-rich river sludge. This study provides a theoretical basis for the S/S treatment of organic-rich river sludge.

1. Introduction

In order to ensure the ecological and navigational function of rivers and harbors, large quantities of river sludge are dredged every year all over the world; in Europe, for example, the amount of sludge dredged is about 200 million cubic meters per year [1,2]. In addition, river pollution has increased over the past two decades due to rapid economic development and urban expansion [3]. Wastewater discharging has resulted in the high levels of pollutants in coastal and river sludge, such as heavy metals and exogenous organic matter [4]. Contamination of heavy metals in river sludge is harmful to humans and the environment. Due to its persistent, non-degradable, and bioaccumulative characteristics, it not only harms aquatic organisms but also causes visceral and nerve damage to human bodies [5]. Therefore, the pollutants in dredged river sludge without proper treatment may cause secondary pollution to the environment [6]. Current disposal strategies for river sludge are mainly pollutant removal or controlling of pollutant release, including incineration, anaerobic digestion, pyrolysis, gasification, thickening, and landfill treatment [7,8,9]. Among them, solidification/stabilization (S/S) treatment is an efficient and low-cost remediation method, which mainly fixes the contaminants in river sludge by using binder materials as stabilizers via chemical solidification and physical encapsulation [10]. Although the pollutants encapsulated in the sludge are not removed, the harmful substances will be transformed into a low-pollution, low-solubility, and chemically stable state, thus significantly reducing their potential hazards. S/S treatment is now recognized as one of the most effective and feasible pre-treatment methods for the treatment and resource utilization of river sludge [11].
Common silicate cement is widely used as a curing agent in S/S treatment of river sludge due to its strong solidification properties and low price. However, on the one hand, every ton of cement production emits 1000 kg of CO2, leading to a huge burden on the environment [12]. On the other hand, organic matter in the sludge interferes with the hydration and cementation process of cement, resulting in a poor solidification effect [13]. Therefore, it is necessary to find more low-carbon and effective alternative gel materials for organic sludge. Current alternative gel materials are mainly industrial wastes or by-products, natural volcanic ash, and reactive minerals with hydraulic or volcanic ash properties [14]. Among them, phosphogypsum and blast furnace slag are ideal alternative materials [15]. Phosphogypsum is a solid by-product from the treatment of phosphate ore with sulfuric acid in phosphoric acid production. Its main component is calcium sulfate-containing crystalline water and a variety of impurities such as silica, phosphorus, fluorine, and organic matter. It is one of the most studied solid wastes in the environmental protection field. Global annual emission of phosphogypsum is about 120 million tons. It is usually used in the construction industry for the production of cement retarders, gypsum boards, gypsum blocks, and other building materials [16]. But only 15% of phosphogypsum is recycled, and the rest is usually piled up in coastal areas, causing a slight impact on the environment under weathering. For example, in the Piney Point Stack area of Florida, the cost of repairing phosphogypsum pollution is about USD 300 million [17]. Blast furnace slag is a by-product produced during the smelting of pig iron in blast furnaces, and its main components include silicon dioxide (SiO2), alumina (Al2O3), calcium oxide (CaO), etc., which are similar to the components of cement. In the S/S process of sludge, heavy metals are fixed, and the strength of solidified sludge is enhanced through ion exchange, water and gel formation, and chemical bond transformation [18]. Lucas’ research shows that compared to concrete pavement, blast furnace slag concrete pavement reduces CO2 emissions by about 48% and energy consumption by about 37%. In addition, in terms of economy, using blast furnace slag instead of 60% cement reduces the cost of 1 m3 pavement by 16.28% [19]. Utilization of phosphogypsum and blast furnace slag to replace cement reduces the impact of waste on the environment and saves resources, which has important economic and environmental benefits.
In addition to the gel material, the rich organic matter in sludge is also a major issue in its S/S treatment. Previous studies show that the strength of cementitious sludge decreases with the increase in organic matter content when organic-rich sludge is solidified [20]. Abundant organic matter may affect the hydration reaction of cement. According to survey data, the organic matter content in river sludge is usually within 5–10%, with some urban river sludge having an organic matter content greater than 10%, which is relatively high. Therefore, silt with organic matter content in the range of 5–10% in the river is defined as medium organic matter sludge, and sludge with organic matter content above 10% is defined as high organic matter sludge [21,22]. Organic matter can be divided into humus and non-humus. Non-humic substances refer to partially decomposed plant and animal residues in sediment, which are structurally simpler compared to humic substances. Due to their faster decomposition rates, they are typically present during the early to middle stages of organic matter decomposition and exist for shorter periods. Humus is the organic matter that is decomposed and transformed by microorganisms, containing a variety of functional groups, such as carboxyl, phenolic hydroxyl, alcohol hydroxyl, and carbonyl groups. They have an important effect on the physical properties and microbial activity of soil and sludge. According to the difference in their solubility in acids and alkalis, they can be further classified into humic acid (HA), fulvic acid (FA), and humin [23]. HA and FA are the soluble components of humus; humin is the insoluble and relatively stable component. They have significantly different influences on the S/S treatment of sludge. Among them, the soluble components have a greater effect on the solidified sludge, and the related studies were focused on HA and FA. FA is the water-soluble part of humic acid with the smallest molecular weight and the highest content of reactive groups, and it is soluble in different pH ranges. HA, with a much larger molecular weight, is insoluble at neutral and acidic pHs. In addition to the differences in molecular structure, there are also differences in their effects on solidified sludge. Previous studies show that both FA and HA weakened the hydration reaction of cement, but FA possesses a higher content of oxygen-containing functional groups and is more soluble in neutral and alkaline conditions. Therefore, FA has a stronger weakening effect on the strength of solidified sludge than HA [24].
At present, the S/S of organic-rich river sludge is a major issue in the treatment and recycling of river sludge. Previous studies were mainly focused on the influence of humus content on the solidified sludge; few studies were conducted on the change in humus and its interaction with cement hydration during the S/S process of organic-rich sludge. In addition, due to the fact that the S/S curing time of sludge not only affects the final solidification effect but also relates to economy, construction efficiency, and maintenance costs, Wang et al.’s research shows that after 28 days of curing, the unconfined compressive strength and heavy metal leaching of sludge have achieved good results [25]. However, considering the impact of organic matter on hydration reactions, the curing time should be extended. Therefore, this study selected the river sludge with medium and high content of organic matter for the S/S treatment with modified curing agent (GCP) and cement (P.O). This study detected the changes in the properties of solidified sludge and humus during the curing process from day 1 to day 50 and explored their interactions.

2. Materials and Methods

2.1. Experimental Materials

Two kinds of sludge with different contents of organic matter were selected. They were collected from 0 to 20 cm depth of the river by a Petersen grab sampler. The sludge with high content of organic matter (SHOC) was collected from the Sanyang Wetland (Wenzhou, China). The sludge with a content of medium organic matter (SMOC) was collected from the Wenruntang River (Wenzhou, China). The basic properties of the sludge are shown in Table 1. The contents of heavy metals in the sludge are shown in Table 2; the concentrations of As and Ni exceeded the minimum risk concentration of the national standard of China (Soil Pollution Risk Control Standard for Construction Land, GB36600-2018). Figure 1 presents the FTIR spectra of the raw sludge. The spectral profiles of high organic matter and medium organic matter sediments exhibit consistent trends, indicating similar chemical structures in their core organic components.
Two solidifying agents (GCP and P.O) were used for the S/S treatment of the sludge. GCP was the composite solidifying agent, with the main components of cement, slag, and phosphogypsum. The P.O was 42.5# ordinary silicate cement purchased from Wuhan Huaying New Century Environmental Protection Equipment Co., Ltd. (Wuhan, China). The main chemical compositions of the two curing agents are shown in Table 3.

2.2. S/S Treatment of Sludge

The treatment of sludge was divided into four groups as shown in Table 4. After the sludge was collected, it was statically placed for 24 h to remove the overlying water. A total of 10 g of the remaining sludge was taken to determine its water content after mixing. Then, a filter press was used to reduce the water content of the sludge to about 90 ± 5%. After the sludge dewatering, 150 m3/kg of curing agent was added and mixed with a cement mortar mixer (JJ-5 type, Yueda). After mixing, the sludge samples were filled into the molds (a diameter of 3.91 cm and a height of 8.0 cm) in several batches and were repeatedly vibrated to exclude air bubbles from the samples, and the surface of the samples was smoothed with a soil scraper after filling the molds. Finally, they were placed in a curing chamber and solidified for 2 d at 20 ± 2 °C and a humidity of 95 ± 2%. Subsequently, the surface film of the samples was removed, and the solidification process was continued for 1, 2, 3, 5, 7, 14, 28, and 50 d. Three parallels of each sample were set up in each group to enhance the accuracy and repeatability of the experiment.

2.3. Analytical Methods

2.3.1. Physical and Chemical Properties

The water content of the sludge was determined using the weight method (HJ613-2011); the sludge samples were dried to constant weight in an oven (DHG-9055A, Yiheng, China) at 105 ± 5 °C, with mass measurements before and after drying performed using an analytical balance (ATX224R, Shimadzu, Japan). The moisture content was calculated from the mass difference. Organic matter was determined using the scorching reduction method (HJ761-2015); the oven-dried samples were subsequently combusted in a muffle furnace (MF-1200C-M, BOECO) at 600 ± 20 °C for 3 h. The organic matter content was determined by the mass loss of the dried samples after combustion at 600 ± 20 °C. The pH was detected using the potentiometric method (HJ962-2018), and deionized water was added as the extraction agent at a water-to-soil ratio of 2.5:1 (w/w). After thorough mixing and standing, the pH value was measured using a calibrated pH probe (HQ40, Hach, USA). Zeta potential was measured using a zeta potential analyzer (SZ-100Z2, Horiba, Japan) at its corresponding pH conditions. The content of heavy metals in the sludge was determined by inductively coupled plasma mass spectrometry (HJ1315-2023), and following digestion on a hot plate (DB-3EFS, Lichen, China), the samples were analyzed using inductively coupled plasma mass spectrometry ICP-MS (ICAP TQ, Thermo Fisher, USA).

2.3.2. Mechanical Properties

When the sludge samples were collected at different S/S times, their size and mass were measured using vernier calipers and a balance. Then, a strength test was conducted using a press. The samples were placed on the bottom plate of the unit, and the wheel was turned to lower the top plate. When the upper plate was in contact with the top surface of the specimen, the pressure gauge was set to zero. An external force was then applied to compress the specimen to failure.

2.3.3. Extraction of FA and HA

Determination of FA and HA in stabilized sludge humus using a modified Kumada method: A total of 5 g of sieved sample was taken with the addition of distilled water and uniform stirring. The sample was extracted in a constant-temperature oscillating water bath (SHA-CB, VRERA) at 70 ± 2 °C for 1 h, followed by centrifugation at 3500 r min−1 for 15 min using a high-speed centrifuge (H1750R, Xiangyi, China). The sediment was taken with the addition of a mixture of NaOH and Na2P2O7. The mixture was continually extracted for 1 h in the same oscillating water bath at 70 ± 2 °C and centrifuged at 3500 r min−1 for 15 min again. The supernatant was taken, and the pH was adjusted to 1.0–1.5 using 0.5 mol L−1 H2SO4. The solution was held at 60–70 °C for 1.5 h and left overnight, and the solution was filtered and fixed in a 50 mL volumetric flask on the next day to obtain FA. The precipitate on the filter paper was washed with H2SO4 and then dissolved in warm NaOH into a 50 mL volumetric flask and fixed in distilled water to obtain HA.

2.3.4. Characterizations of Microscopic Properties

A scanning electron microscope (Merlin Compact, Zeiss, Germany) was used to characterize the microstructure of the solidified samples. The mineral composition of the raw sludge and solidified sludge was analyzed by X-ray diffraction (D8ADVANCE, BRUKER, USA). The samples were crushed and placed in the slots of slides for compaction and then placed on the carrier table. The corresponding diffraction intensities at 2θ were collected by X-ray emission at a scanning rate of 5°/min, with a scanning start angle of 10° and an end angle of 80°. The obtained data were analyzed using MDI Jade 6.0 and the PDF standard card database for physical phase analysis. The characteristic vibration bands of the solid samples were obtained by Fourier-transform infrared spectroscopy (FTIR), with scanning performed on a Fourier-transform infrared spectrometer (TRENSOR 27, BRUKER, USA) over the wavenumber range of 4000–400 cm−1.

3. Results and Discussion

3.1. Subsection

Compressive strength is an important mechanical property of solidified materials, which directly relates to the structural bearing capacity and deformability of the materials [26]. The solidified sludge is mostly applied in construction materials, road bases, river dams, slope protection, and other projects; therefore, the solidified sludge must have certain compressive strength.
Figure 2 shows the changes in compressive strength of SMOC and SHOC during the S/S treatment with different curing agents. The compressive strength of the solidified sludge in each group increased with the increase in the S/S treatment time, and its increasing rate greatly varied with the content of organic matter and the composition of curing agents. The increasing rate of compressive strength of the solidified sludge could be divided into three stages within 1–50 d. The first stage was the initial 7 d of the S/S treatment; the compressive strength increase of the sludge in each group was weak. The fastest strength increase was observed in group H-GCP, with only 180.89 KPa of the increase in the initial 7 d. The reason may be that the strength of the solidified sludge comes from the hydration reaction of the 2CaO·SiO2, 3CaO·SiO2, and 3CaO·Al2O3 in the cement, generating hydrated calcium silicate gel (C-S-H) and hydrated calcium aluminate gel (C-A-H). These hydration products wrapped around the surface of the sludge particles and filled in the pores, thus increasing the density and compressive strength of the solidified sludge [1]. However, humus inhibited the hydration reaction of the solidified sludge and delayed the generation of hydration products, thus reducing the strength of the solidified sludge [27]. In addition, a previous study showed that the surface of the soil matrix with a high content of organic matter carried a large amount of negative charge, hindering the mutual contact between the solidifier and sludge particles and reducing the early S/S reaction rate of the solidified sludge [13]. As a result, the hydration products of the solidified sludge were reduced, and a porous structure was formed to further reduce the density of the solidified sludge. The second stage of S/S treatment was 7–28 d. The strength-increasing rate of the solidified sludge at this stage was significantly enhanced, and the higher increasing rate of the GCP-solidified sludge was obtained. In addition, the strength of the solidified SHOC was significantly higher than that of the solidified SMOC. The strengths of H-GCP and H-P.O were 1628.03 and 1345.22 Kpa, respectively, which met the strength of cement-stabilized materials for road subgrade (class II of the Technical Rules for Construction of Highway Pavement Subgrade, JTGTF20-2015). Although the organic matter generated a hindering effect on the solidification of the sludge, the increase in organic matter content was not linearly related to the decrease in the strength of solidified sludge, but there was a limit effect on the content of organic matter [27]. A previous study showed that when the organic matter content exceeded 7.7%, the decrease in compressive strength with the increase in organic matter content was not obvious [28]. Therefore, the difference in strength between the solidified SMOC and SHOC in this study may not be dominated by organic matter content but by the changes in the physical properties (e.g., porosity and microstructure) of the sludge caused by the addition of curing agents. The third stage was 28–50 d, and the increasing rate of the strength of the solidified SHOC was slowed down, while that of the solidified SMOC was accelerated. The maximum compressive strength of H-GCP and H-P.O reached 2242.04 and 1156.69 Kpa at 50 d, respectively. It was demonstrated that the presence of slag and phosphogypsum in the curing agent could improve the solidification effect on SHOC.

3.2. Water Content and Mass Change of Solidified Sludge

Water content has a significant effect on the mechanical properties of solidified sludge. Lower water content is conducive to increasing the unconfined compressive strength of solidified sludge. In addition, the reduction in water in the S/S process means that more cementing substances can fill the pores between soil particles, thus increasing the compactness and strength of solidified sludge.
Figure 3 shows the changes in water content and unit mass of solidified sludge during the S/S process. It can be seen that the water content of the solidified sludge in each group showed a brief increase and then a continuous decrease with the increase in S/S time, finally reaching a stable stage after 28 d. Among them, the solidified SHOC maintained a high water content in the initial stage, possibly due to the high hydrophilicity of the organic matter components, hindering the hydration reaction in the solidified sludge. Then, the water content of the solidified SHOC significantly decreased in 7–14 d and gradually reached a stable state after 28 d. The water content was lower than that of the solidified SMOC in the same period, which was consistent with the changing trend of the compressive strength of the solidified sludge in Figure 2. This result indicates that the solidified SHOC experienced a strong hydration reaction stage from 7 to 14 d, which produced more hydration products and faster strength growth. In addition, the higher water content in the solidified SHOC within this stage also accelerated the hydration reaction, which resulted in a significantly higher increase in compressive strength than that in the solidified SMOC. However, as the S/S time increased, the hydration reaction slowed down due to water consumption, resulting in a gradual decrease in the increasing rate of strength [20]. At 28–50 d, the water content of the solidified SMOC was 10.09–5.21% higher than that of the solidified SMOC; thus, the strength-increasing trend of the solidified SMOC was more obvious in the late S/S period.
The solidification effect between different groups can be compared by the loss of unit mass of the solidified sludge. The mass of the solidified sludge gradually decreased with the increase in S/S time, and the mass loss of GCP-solidified sludge was smaller than that of P.O-solidified sludge. It may be due to the fact that the phosphogypsum and blast furnace slag in GCP can form the high-strength AFt during the solidification process, which can significantly improve the strength of the solidified sludge, reduce the pore size, and stabilize the pore structure. In addition, the content of active volcanic ash content in blast furnace slag can also form Ca(OH)2, which further enhances the stability of the solidified sludge [29]. Since the same molds were used for the sludge solidification, a higher mass of the solidified sludge at the beginning of the S/S treatment represented a higher density, demonstrating that the solidified SHOC in Figure 2 possessed a higher initial strength.

3.3. Internal and External pH of the Solidified Sludge

The pH value affects chemical reactions in the S/S process of sludge, including hydration reactions [30] and ion exchange processes [31]. In addition, pH has a significant impact on microbial growth and metabolic activity [32]. This in turn affects the formation and properties of solidified sludge. In order to explore the interaction between cement hydration reaction and humus, the internal and external pH of the solidified sludge was monitored at different S/S times (Figure 4).
Figure 4 shows that the internal and external pH of the solidified sludge showed an initial increasing trend, followed by a decrease with the increase in the S/S time. In the initial S/S period, the pH of the solidified sludge exhibited an obvious fluctuation. It showed a slight increase and then decrease in the initial 5d, with a changing range of 0.07–0.3. In 5–7 d, a significant decrease was observed in all groups. The largest decrease was found in group H-P.O, and the pH decreased from 12.41 to 11.93, which corresponds to Figure 5. Subsequently, the largest changes in the pH of all groups were observed at 7–14 d, and the order of pH changes was H-GCP (0.49) > H-P.O (0.40) > M-GCP (0.39) > M-P.O (0.24), which also corresponds to the obvious increase in compressive strength of the solidified sludge in Figure 2. The pH of the solidified sludge exhibited a gradual decrease after 14 d. On the one hand, the hydration reaction consumes OH− and generates stable hydration products such as C-S-H and C-A-S-H, which increases the strength and reduces the alkalinity of the solidified sludge. On the other hand, the organic matter within the solidified sludge decomposes, slowly releasing humus to neutralize OH−, which further reduces the alkalinity of the solidified sludge and the generation amount of calcium silicate hydrate gel. In addition, humic acid reacts with calcium ions and slaked lime, producing insoluble calcium-based humic acid. This insoluble substance can be encapsulated on the surface of cement and sludge particles, thus interfering with hydration and volcanic ash reactions and further inhibiting the solidification of the sludge [33]. When the released humic acid is consumed, the alkaline environment will continue to improve the decomposition of organic matter to produce more humic acid, releasing more H+ to decrease the pH. The alkalinity of the P.O-solidified sludge was significantly higher than that of the GCP-solidified sludge, therefore leading to the release of more humic acid from the sludge. The reason may be that the volcanic ash reaction is produced by the slag and phosphogypsum components of GCP, which consume calcium hydroxide (a cement hydration product). Thus, the hydration reactions are promoted, and the pH of the solidified sludge was reduced, forming a more environmentally friendly solidified product. In addition, the internal pH of the solidified sludge was higher than the external pH of the solidified sludge. It may be due to the passive carbonation of the solidified sludge. The surface of the solidified sludge is exposed to the air, and the cement slurry can spontaneously produce a carbonation reaction with carbon dioxide. Then, the alkaline components are consumed to reduce the alkalinity of the gel materials, and the strength and stability of the gel materials may be reduced by the carbonation, thus leading to volume expansion, cracking, and breakage of the solidified sludge [34]. It is worth noting that the difference between the internal and external pH of the P.O-solidified sludge significantly increased within 14–28 d, indicating that the outer part of the solidified sludge was more eroded by humus during this stage.

3.4. Changes in Organic Matter and Humus in Solidified Sludge

Figure 5 shows the changes in humus and organic matter in the solidified sludge during the S/S treatment. It can be seen that the total amount of humus in the solidified sludge exhibited an initial fluctuation and then a gradual decrease within 1–14 d. The lowest value of all groups was achieved at 14 d, with the order of group H-GCP (109.74) > H-P.O (103.52) > M-P.O (87.15) > M-GCP (83.41). Subsequently, it showed a continual increase after 14 d. The changing trend of humus content corresponded to the pH-changing trend of the solidified sludge in Figure 4, which also demonstrated that the hydration reaction produced an antagonistic effect with humus decomposition. Through comparison, it was found that the overall content of humus in high organic matter solidified sludge was higher than that in medium organic matter solidified sludge within 50 days before solidification, with the main difference being the HA content. However, the effect of humus on cement hydration was mainly from FA, which possesses higher total acidity and carboxyl and phenolic hydroxyl groups than HA [35]. FA also has a higher content of oxygen-containing functional groups, resulting in a stronger binding capacity with calcium and aluminum in the cement hydration process. Therefore, FA has a more obvious inhibition effect on the cement hydration process. In this study, although SHOC had a higher content of organic matter, its FA content was not significantly higher than that of SMOC. Thus, the organic matter in SHOC did not generate more inhibitory effect than that in SMOC on the hydration process and the compressive strength of the sludge. Moreover, the ratio of HA to FA (HA/FA) also decreased from 0.61 to 0.76 at 1 d to 0.28 to 0.42 after 14 d, indicating the increased proportion of FA in the total humus, which corresponded to the gradual decrease in the increasing rate and range of the compressive strength of the solidified sludge in Figure 2. This is consistent with the experimental results of a previous study [20].
The changing trend of organic matter in the solidified sludge (Figure 5) was similar to that of the pH. The content of the organic matter in the solidified sludge also showed a fluctuation in the initial period of S/S treatment. Especially, the content change in organic matter in the P.O-solidified sludge (6.47%) in the initial 5 d was higher than that of the GCP-solidified sludge (6.21%). It may be due to the fact that the release of humus from the solidified sludge is influenced by the composition of curing agents or the rate of chemical reaction, resulting in different consumption rates of organic matter and release rates of humus. The stronger alkalinity of the P.O-solidified sludge promotes the release of more humus to neutralize with free OH−, resulting in a decrease in pH. This result corresponded to the decrease in external pH of the P.O-solidified sludge.
In order to further evaluate the effect of changes in humus on the solidified sludge, the changing amount of the humus in the solidified sludge during S/S treatment was analyzed (Figure 6). The changing amount of the humus exhibited large differences at different stages of S/S treatment. In 1–7d, the total changing amounts of the FA were 22.92 (H-GCP), 14.04 (H-P.O), 42.27 (M-GCP), and 22.88 g/kg (M-P.O), exhibiting small and increasing changes. The changing amounts of the HA were 165.92 (H-GCP), 196.25 (H-P.O), 171.26 (M-GCP), and 127.42 g/kg (M-P.O), showing large and fluctuating changes. It indicates that HA exhibits the highest activity in the initial stage of S/S treatment, mainly neutralizing with the cement hydration products [36]. The hydration products are dissolved under acidic conditions, resulting in a slow increase in the compressive strength of the solidified sludge and a significant fluctuation in pH in the initial stage. The initial increase in HA may be derived from the conversion of FA [37]. When the FA content is high enough, the FA molecules will react with themselves to produce HA, thus continuously supplementing the HA consumed by the neutralization reaction. However, both FA and HA are consumed to a certain extent as the S/S treatment continues; the humus content is less than the production of hydration products at 7–14 d. Thus, the strength of the solidified sludge rapidly increased, whereas the release of new humus from the sludge after 14 d may continue to inhibit cement hydration reactions, resulting in a slow increasing rate of the strength of the solidified sludge.

3.5. Microstructure of Solidified Sludge

The microstructure of the solidified sludge after 1, 14, and 50 d of the S/S treatment was characterized by SEM (Figure 7). The amount of cement hydration products and the denseness of the microstructure can reflect the degree of hydration reaction within the solidified sludge. Figure 7a–d show that the surface structure of solidified sludge is loose, with many pores and cracks (rectangular part composed of dashed lines), indicating a weak connection between soil particles and an overall loose structure. At the 14th day of solidification (Figure 7e–h), it can be observed that the solidified sludge indicates the formation of hydration products, such as a partial cementitious matrix (rectangular portion composed of solid lines) and a needle-shaped ettringite (AFt) within it (elliptical portion). These hydration products can improve the strength of the sludge by enhancing the cementation structure of the sludge particles and stabilize the toxic elements through encapsulation and chemical fixation [24]. As the S/S treatment continued, at day 50, the amount of hydration products in the sludge significantly increased, attached to soil particles or between pores, making the structure denser and the pores significantly reduced. This also corresponds to a significant increase in the compressive strength of the solidified sludge. In addition, the microstructure of the solidified sludge with different curing agents exhibited significant differences, and the number of gels in the GCP-solidified sludge was much higher than that in the P.O-solidified sludge. The main hydration product in the P.O.-solidified sludge was C-S-H gel, which usually accounted for 60–70% of the hydration products, and it could solidify the slurry through the formation of a gel network by electrostatic action [38]. In contrast, the presence of phosphogypsum and slag in the GCP-solidified sludge improved the properties of cement-based materials. In addition, the introduced Al3+ was easily incorporated into the C-S-H gel to promote the generation of calcium–aluminum–silicate hydrate (C-A-S-H) gel [39]. The dreierketten chain of C-A-S-H was longer than that of C-S-H, which contributed to the increase in cross-linking density and the overall stability of the C-A-S-H gel, which resulted in better mechanical properties of the solidified sludge with [40]. An appropriate amount of SO3 addition could accelerate the hydration of C3S and improve the amount and composition of the C-S-H gel, thus further enhancing the strength of the solidified sludge.

3.6. Mineral Composition of Solidified Sludge

The mineral composition of the solidified sludge in each group was analyzed by XRD at 1, 14, and 50 d (Figure 8). It can be seen that the solidified sludge contains various crystals, with the main mineral components being quartz (SiO2; PDF #97-003-9830), calcite (CaCO3; PDF #00-005-0586), potassium feldspar (K(AlSi3O8); PDF #97-001-0274), sodium feldspar (NaAlSi3O8; PDF #97-002-6248), and kaolinite (Al4(OH)8 (Si4O10); PDF #97-006-3315). The intensity and clarity of peak value can reflect the reaction degree of raw materials and the formation of geopolymer gel [41]. In addition, the peaks at 17.9° and 18.9° in the spectrum, as well as at 29.4°, 32°, and 50°, can be considered as part of the cementitious matrix [42,43]. Compared with the P.O-curing agent, the GCP-curing agent undergoes a transformation from C-S-H to C-A-S-H due to the addition of Al, resulting in an increase in the clarity and quantity of diffraction peaks, as well as an increase in the 2θ angle [44]. As the curing time increased, no new diffraction peaks appeared in the sample; only the peak values changed, indicating that the main products had already formed in the early stage of curing. Among them, the diffraction peak intensity of quartz decreased significantly, while the diffraction peak of hydration products also increased to varying degrees, proving the continuation of the hydration reaction. In addition, a similar lattice was found in the GCP- and P.O-solidified sludge, and obvious formation of new minerals was not observed, suggesting that the enhancement of the mechanical properties of the solidified sludge is mainly due to the binding effect of the geopolymer gels [45].

4. Conclusions

This paper conducted S/S treatment for river sludge with medium and high content of organic matter by using two different curing agents. The changes in compressive strength, water content, mass, pH, organic matter, and humus of the solidified sludge were investigated, and the interrelationships were also explored. The following conclusions were drawn:
  • The presence of humus in organic matter of sludge could affect the compressive strength of the solidified sludge, resulting in a slow increasing rate of the strength of the solidified sludge in the initial stage. The content of organic matter was not a major factor for the large difference in compressive strength between SHOC and SMOC. Curing agents exhibited a stronger influence than organic matter. GCP exhibited a stronger solidification effect on organic-rich sludge due to the blast furnace slag and phosphogypsum, achieving a higher compressive strength of its solidified sludge (2242.24 Kpa) at 50 d than that of P.O-solidified sludge, which was also supported by microstructure analysis.
  • The water content of the solidified sludge was affected by the hydration reaction. It stabilized in a certain range after 14 d, which reflected the solidification level of the solidified sludge. The mass of the solidified sludge gradually decreased with the increase in S/S treatment time, and a higher mass loss corresponded to a lower solidification level.
  • The pH of the solidified sludge exhibited a fluctuation trend, followed by a continual decrease with the continual increase in the humus content. The external pH was always lower than the internal pH due to the influence of the carbonation reaction. The P.O-solidified sludge displayed more alkalinity and lower stability.
  • The changing trend of humus content in the solidified sludge was opposite to the pH, indicating that the hydration reaction was antagonistic to the decomposition of humus. The main difference in organic matter between the solidified SHOC and SMOC was the content of HA. Neutralization of humus with the hydration products resulted in the fluctuation of pH. The humus content in the solidified sludge was negatively correlated with the compressive strength.

Author Contributions

Conceptualization, Y.Z. and F.R.; methodology, Y.Z. and F.R.; software, Y.Z. and F.R.; validation, S.L.; formal analysis, Y.Z. and F.R.; investigation, Y.Z. and F.R.; resources, Y.Z. and F.R.; data curation, Y.Z. and F.R.; writing—original draft preparation, Y.Z. and F.R.; writing—review and editing, L.W. and C.F.; visualization, S.L.; supervision, L.W. and C.F.; project administration, Y.Z. and F.R.; funding acquisition, L.W. and C.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Key R&D Program of China (No. 2022YFE0105000).

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to funder restrictions.

Acknowledgments

The authors express their sincere gratitude for the work of the editor and the anonymous reviewers.

Conflicts of Interest

The authors declare no conflicts of interest.

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Water 17 01153 g001
Figure 1. FTIR image of the original sludge. (H: high organic matter sludge, M: medium organic matter sludge.)
Figure 1. FTIR image of the original sludge. (H: high organic matter sludge, M: medium organic matter sludge.)
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Figure 2. Changes in compressive strength with S/S treatment time. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 2. Changes in compressive strength with S/S treatment time. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Figure 3. Water content and unit mass of the solidified sludge. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 3. Water content and unit mass of the solidified sludge. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Figure 4. Changes in internal and external pH of the solidified sludge. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 4. Changes in internal and external pH of the solidified sludge. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Figure 5. Content of humus (FA, HA) and organic matter with S/S time. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 5. Content of humus (FA, HA) and organic matter with S/S time. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Figure 6. Changes in humus substances in the solidified sludge: (a) FA; (b) HA. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 6. Changes in humus substances in the solidified sludge: (a) FA; (b) HA. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Figure 7. Microstructure of the solidified sludge during S/S treatment. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 7. Microstructure of the solidified sludge during S/S treatment. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Figure 8. XRD images of solidified sludge: (a) 1 d; (b) 14 d; (c) 50 d. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
Figure 8. XRD images of solidified sludge: (a) 1 d; (b) 14 d; (c) 50 d. (H-GCP: high organic matter sludge + GCP, H-P.O: high organic matter sludge + P.O, M-GCP: medium organic matter sludge + GCP, M-P.O: medium organic matter sludge + P.O).
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Table 1. Basic properties of river sludge.
Table 1. Basic properties of river sludge.
SludgeMoisture Content (%)pHOrganic Matter Content (%)Zeta Potential (mv)
SHOC228.57.812.9−40.5
SMOC235.87.28.3−25.7
Table 2. Heavy metal content of river sludge (mg/kg).
Table 2. Heavy metal content of river sludge (mg/kg).
SludgeCrAsNiCuZnPb
SHOC393.8807.7190.9297.71545.3172.7
SMOC670.0751.5357.5361.52065.8175.6
Table 3. Chemical composition analysis of curing agent (%).
Table 3. Chemical composition analysis of curing agent (%).
Curing Agent CaO SiO2 SO3 Al2O3 MgO Fe2O3 K2O Na2O
GCP 51.3 18.1 13.0 7.9 4.8 3.0 0.6 0.3
Cement 66.2 17.5 4.1 4.7 2.1 3.8 0.8 0.6
Table 4. Group settings.
Table 4. Group settings.
Sample Name Organic Matter Content (%) Curing Agent S/S Time (d)
H-GCP 12.9 GCP 1, 2, 3, 5, 7, 14, 28, 50
H-P.O 12.9 P.O 1, 2, 3, 5, 7, 14, 28, 50
M-GCP 8.3 GCP 1, 2, 3, 5, 7, 14, 28, 50
M-P.O 8.3 P.O 1, 2, 3, 5, 7, 14, 28, 50
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Zhu, Y.; Ran, F.; Liu, S.; Wang, L.; Fan, C. Effects of Humus and Solidification Agents on the Solidification/Stabilization Process of Organic-Rich River Sludge: Characteristics of the Stabilized Sludge. Water 2025, 17, 1153. https://doi.org/10.3390/w17081153

AMA Style

Zhu Y, Ran F, Liu S, Wang L, Fan C. Effects of Humus and Solidification Agents on the Solidification/Stabilization Process of Organic-Rich River Sludge: Characteristics of the Stabilized Sludge. Water. 2025; 17(8):1153. https://doi.org/10.3390/w17081153

Chicago/Turabian Style

Zhu, Yuqi, Fuyuan Ran, Sihong Liu, Liujiang Wang, and Chunzhen Fan. 2025. "Effects of Humus and Solidification Agents on the Solidification/Stabilization Process of Organic-Rich River Sludge: Characteristics of the Stabilized Sludge" Water 17, no. 8: 1153. https://doi.org/10.3390/w17081153

APA Style

Zhu, Y., Ran, F., Liu, S., Wang, L., & Fan, C. (2025). Effects of Humus and Solidification Agents on the Solidification/Stabilization Process of Organic-Rich River Sludge: Characteristics of the Stabilized Sludge. Water, 17(8), 1153. https://doi.org/10.3390/w17081153

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