Simulation of Heavy Metal Removal in Irrigation Water Using a Shell-Derived Biochar-Integrated Ecological Recycled Concrete

Abstract

Water pollution intensifies water scarcity and poses a significant threat to ecosystems and human health. Construction waste generated by rapid urbanization also imposes a considerable burden on the environment. Fortunately, a large portion of this waste can be efficiently converted into recycled aggregates and reused in various fields including environmental remediation. In this study, three types of eco-recycled concretes (ERC) (Control-ERC, Biochar-ERC-1, and Biochar-ERC-2) were formulated by integrating shell-derived biochar with recycled aggregates. The porosity and water permeability of these concretes were characterized, and their efficacy evaluated in treating polluted water with six primary heavy metals (HMs), i.e., cadmium (Cd), chromium (Cr), arsenic (As), manganese (Mn), lead (Pb), and copper (Cu). Biochar addition significantly enhanced the continuous porosity and water permeability of the concrete, and substantially enhanced its adsorption capacity of HMs. Specifically, Biochar-ERCs removed over 90% of As, Cd, and Mn, and achieved a removal rate exceeding 60% for other HMs, surpassing the performance of Control-ERC. This study not only lays a solid foundation for the wide application of Biochar-ERCs in the field of environmental protection and remediation, but also provides strong technical support and practical examples for advancing the circular economy model of converting waste into resources while addressing the challenge of global water scarcity.

1. Introduction

Water stress, exacerbated by improper usage, environmental pollution, and climate change, poses significant threats to both the natural environment and human health [1,2]. The extensive use of water for irrigation has led to severe water scarcity. Even when water is available for irrigation, it is often insufficient or contaminated, thereby adversely affecting the yields and the quality of agricultural production [3]. Furthermore, polluted irrigation water can contaminate soil, leading to long-lasting detrimental effects on the ecosystem and human health [4]. Pollution by heavy metals (HMs) in the environment is a major concern due to their non-degradability and potential toxicity to both ecosystems and humans [5]. The regeneration of clean water is a lengthy process through the natural water cycle, which has been further complicated and hindered by human activities [6]. Therefore, reusing treated wastewater or used water is imperative, which necessitates effective technologies for water pollution control and treatment. However, the high initial concentrations of HMs and the costs associated with these technologies require a considerable investment to improve the quality of treated wastewater to a level for direct reuse in irrigation or landscaping. Among the conventional and innovative technologies, adsorption stands out as one of the most efficient methods for removing HMs from wastewater [7].

Biochar, characterized by its large specific surface-area-to-volume ratio, abundant pore space, high carbon content, and various surface functional groups, has emerged as a novel material with widespread applications in environmental remediation. It has proven to be effective in soil quality improvement, pollutant removal, carbon sequestration, and emission reduction [8,9]. This versatile material can be produced from a wide range of raw materials, including plant residues and organic solid waste [10,11], in alignment with the prevalent global resource policy that advocates conservation and reuse. Similarly, concrete, an important component in construction projects, is produced in large quantities, leading to the emergence of recycled concrete as a sustainable alternative [12]. Recycled concrete is obtained from previously used concrete, such as construction waste. Its reprocessing and reuse significantly mitigate resource wastage, facilitate concrete recycling, and propel the advancement of green environmental protection [13]. Furthermore, it aids in reducing land contamination, adsorbs HMs ions, and plays a pivotal role in environmental restoration [14]. By incorporating biochar and other materials with a multi-void structure into recycled concrete, the adsorption performance of pollutants is enhanced, thus augmenting the environmental remediation efficacy of the concrete. This approach exemplifies the principle of utilizing waste to manage waste [15,16], thereby promoting a sustainable and eco-friendly solution. The addition of biochar to the concrete production process not only minimizes the requirement for clinker in cement production process but also reduces the carbon dioxide (CO₂) emissions associated with clinker fabrication. Furthermore, biochar, distinguished by its high porosity and extensive specific surface area, exhibits the potential to sequester atmospheric CO₂ within the concrete structure [17,18]. High-strength lightweight concrete, formulated using biochar aggregate derived from kitchen waste digestate, effectively reduces the carbon emission of construction materials [19]. In addition, biochar enhances multiple critical properties of concrete, including compressive strength, flexural strength, and split tensile strength [13,20]. Moreover, biochar produced from lignocellulosic biomass in conjunction with sustainable eco-permeable concrete formulated with modified corn cob coarse aggregate has been shown to improve water retention within concrete [19,21], thereby significantly enhancing the strength and durability of the eco-concrete. The integration of biochar into concrete not only facilitates the recycling of concrete but also promotes the utilization of straw and organic solid waste as the sources of biochar. Furthermore, this combination demonstrates remarkable removal efficiency for manganese (Mn), nitrate (NO₃⁻), total phosphorus (TP), and organic pollutants [22], which enhances the environmental remediation effectiveness of both concrete and biochar. At present, the integration of marine-derived biochar with recycled concrete for environmental remediation is still in the stage of technical exploration and initial application. This composite can improve the elastic recovery characteristics and durability of biochar within the cementitious environment [23]. The incorporation of carbon dioxide (CO₂)-storing algal biomass and biochar with concrete effectively reduces CO₂ emissions while effectively adsorbing HM ions and reducing their contents [24,25]. Additionally, the combination of marine sediments with concrete not only reduces carbon emissions and enhances the immobilization of HMs, but also offers potential economic benefits for the treatment and utilization of oceanic sediments [26]. By investigating the application of marine-derived biomass, such as shells, in combination with concrete, this study endeavors to ascertain its efficacy in removing HMs from irrigation water. Consequently, it introduces a novel and sustainable solution for environmental remediation. This innovative method leverages the natural properties of marine biomass and recycled concrete to provide a cost-effective and eco-friendly solution addressing HM contamination in irrigation water, ultimately contributing to the preservation and enhancement of our natural environment.

2. Materials and Methods

2.1. Preparation of Experimental Materials

2.1.1. Preparation of Shell-Derived Biochar

The raw-material shells were collected from the beach at Binhai Park in Qingdao City, Shandong Province, China. These shells were then washed thoroughly with tap water 5 times, baked at 105 °C for 2 h, and subsequently subjected to crushing treatment. After this pretreatment, the shells were accurately weighed and placed into a muffle furnace (OTF-1200X-Ⅲ-S, China). They were purged with nitrogen for 10 min, pyrolyzed at 900 °C for 2 h, and allowed to cool. The resultant material was then ground and passed through a sieve with a mesh size of 10,000. The fine particles were collected and stored in a dry place for later use. A scanning electron microscope (SEM, ZEISS Crossbeam 550, Germany) was used to examine the surface morphology and structure of the biochar.

2.1.2. Production of Eco-Recycled Concrete with Shell-Derived Biochar

In this study, three groups of eco-recycled concrete (ERC) blocks were prepared, namely Control-ERC, Biochar-ERC-1 and Biochar-ERC-2, with three blocks in each group. The raw materials used for the preparation of these concrete blocks included coarse aggregate sourced from construction waste and biochar derived from marine shell. The specific raw material ratios were as follows: 16.6 parts of cement (Shandong Shanshui Cement Group Limited, China); 76.6 parts of recycled aggregate; 0, 0.3, and 0.6 parts of marine-sourced shell-derived biochar, respectively, for Control-ERC, Biochar-ERC-1, and Biochar-ERC-2; 0.4 parts of reinforcing agent; and 6.0 parts of water. Coarse aggregate particles were sized in the range of 5–10 mm. During the mixing process, the aggregate, cement, and reinforcing agent (Zhejiang Kamiao Technology Co., Ltd., China) were placed first, followed by the addition of deionized water. Except for the control group, which did not contain shell-derived biochar, the other two groups incorporated 0.3 and 0.6 mass portions of shell-derived biochar, respectively. During molding, the molds (100 mm × 100 mm × 100 mm) were coated with mold release agent (Quanzhou Osle Industrial Materials Co., Ltd., China), filled with pounded and smoothed concrete, and wrapped with plastic film. The concrete blocks were then cured in the molds for 24 h before being demolded, rewrapped, and placed for an additional week to complete the concrete preparation. Procedural details of producing the concrete can be found in our previous study [16].

2.1.3. Water Permeability and Porosity Characterization of the Eco-Recycled Concrete

The water permeability coefficients of the three types of ERCs were measured using the constant head method [27], and three replicate blocks from each group were selected for the experiments. The experimental configuration is depicted in Figure 1a. First, the space between the test block and the square cylinder unit was sealed using sealing mud. The square cylinder unit was then placed in the drum unit. Water was then introduced, and the flow rate was adjusted to attain the desired head height. The difference in water level was measured with a straightedge and recorded as H. The water temperature (T) was also monitored. A timing duration of 60 s was set, during which a collection bucket was used to collect the water exiting from the outlet.

The permeability coefficient can be expressed by Equation (1):

$${\mathrm{k}}_{\mathrm{T}}={\displaystyle \frac{\mathrm{Q}\times \mathrm{L}}{\mathrm{A}\times \mathrm{H}\times \mathrm{t}}}$$

(1)

where the k_T (mm/s) is the water permeability coefficient of the block when the water temperature is T °C; Q (mm³) is the volume of water seeping out at time t; L (mm) is the thickness of the concrete block; A (mm²) is the upper surface area of the concrete block; H (mm) is the water level difference; and t (s) is the time duration.

After conducting the permeability coefficient experiments, continuous porosity tests of the concrete blocks were performed. At the conclusion of the permeability coefficient experiment, the test block was securely tied with a rope and submerged in a bucket of water, making sure that the test block was completely submerged without touching the bottom, as illustrated in Figure 1b. The submerged test block was then weighed using a handheld electronic scale to obtain M₁. The continuous porosity of the test block was calculated using the following Equation (2):

$${\mathrm{C}}_{\mathrm{v}\mathrm{o}\mathrm{i}\mathrm{d}}=\left[1-{\displaystyle \frac{{\mathrm{M}}_2-{\mathrm{M}}_1}{\mathsf{\rho }\mathrm{V}}}\right]\times 100\mathrm{\%}$$

(2)

where C_void (%) is the continuous porosity; M₁ (g) is the weight of the concrete block when submerged in water; M₂ (g) is the dry weight of the concrete block; ρ (g/cm³) is the density of water; and V (cm³) is the volume of the concrete block.

2.2. Simulation of Wastewater Filtration Process with the Concrete

2.2.1. Simulated Wastewater Preparation

A simulated wastewater effluent (SWW) was prepared with HM concentrations set to the maximum permissible emission concentrations stipulated in the People’s Republic of China’s Integrated Wastewater Discharge Standard [28]. Specifically, the concentrations were set as follows: lead (Pb) at 1.0 mg/L, cadmium (Cd) at 0.1 mg/L, chromium (Cr) at 1.5 mg/L, arsenic (As) at 0.5 mg/L, Mn at 2.0 mg/L, and copper (Cu) at 0.5 mg/L. These concentrations were achieved through the preparation of a solution composed of Pb(NO₃)₂, CdCl₂, K₂Cr₂O₇, Na₂HAsO₂·7H₂O, MnCl₂·4H₂O, CuCl₂·2H₂O, and deionized water.

2.2.2. Filtration Experiments Using ERCs for SWW

A simulated irrigation canal, in the form of an open reactor, was designed to evaluate the filtration efficiency of HMs using ERCs within agricultural irrigation systems. The experimental setup mainly consists of 4 parts, including an acrylic support plate, a partition plate, water purification tank with an ERC block, and purified water tank, as illustrated in Figure 2. The open-top reactor, the partition plate, and the purification water tank were constructed using gold-crystal super-white glass with a thickness of 6 mm. All the external dimensions of each component are illustrated in Figure 2. Three replicate ERC blocks of each type were selected for conducting the parallel experiments to ensure the reliability and reproducibility of the results.

The SWW was slowly introduced into the reactor from the left side of the device. After all of the SWW had been introduced into the device, it was allowed to react for 2 min. Following this period, the partition was then removed to enable the entire volume of the SWW to pass through the ERC block and into the tank on the right side.

The conductivity, pH, and dissolved oxygen (DO) of the SWW before and after filtration were monitored. Samples were collected and acidified using 2% nitric acid, and the concentrations of the HMs (Pb, Cd, Cr, As, Mn, Cu, etc.) in the samples were determined by inductively coupled plasma mass spectrometry (ICP-MS) to determine the adsorption of HMs by the ERCs.

The strengths of concrete blocks were tested with an electro-hydraulic servo universal testing machine after the filtration experiment. The elemental composition was analyzed with SEM–EDS (energy-dispersive spectrometer) with aggregates from crushed concrete blocks.

2.3. Statistical Analysis

One-way ANOVA was used to assess the differences in the adsorption capacity of different ERCs for HM ions in the SWW. The data were presented as mean ± standard deviation, with n = 3 replicates. A statistically significant difference between groups was indicated by a p value of less than 0.05. All statistical analyses were conducted in R (R4.2.2) and figures were produced using the package ggplot2.

3. Results and Discussion

3.1. Characterization of Shell-Derived Biochar

SEM analysis indicated that the particle size of the shell-derived biochar varied from less than 1 μm to 5 μm, and exhibited irregular shapes and morphologies (Figure 3), which could be attributed to the inherent characteristics of the shell material and the pyrolysis process conditions used for their production. This variability in particle size and shape can influence the biochar’s surface area, porosity, and overall performance of biochar-integrated ERCs.

3.2. Characterization of ERCs

The surface morphology of the ERC blocks, as depicted in Figure 4, was captured using a high-resolution scanning device. The image displays a porous structure formed by relatively uniform recycled aggregates with biochar incorporated within, facilitating water permeability and enhancing pollutant absorption capabilities.

Table 1. Physical characterization of the concrete blocks made with recycled aggregates and shell-derived biochar.

Concrete Blocks	Mass (g)	Dimension (cm)	Density (g/cm3)	Permeability Coefficients (mm/s)	Continuous Porosity (%)	Strength (MPa)
Control-ERC	1676.67 ± 20.82	10 × 10 × 10	1.68	19.42 ± 0.14	33.83 ± 1.76	5.64
Biochar-ERC-1	1590 ± 26.46		1.59	23.08 ± 0.76	37.83 ± 0.58	6.53
Biochar-ERC-2	1606.67 ± 20.82		1.61	26.34 ± 0.80	37.17 ± 1.26	5.10

summarizes the fundamental physical characteristics of ERCs.

3.2.1. Permeability Coefficients

Water permeability is the main characteristic for evaluating the performance of pervious concrete [23]. The one-way ANOVA analysis indicate that there was a significant difference in the permeability coefficients of the three types of concrete (p < 0.01) (Table 1, Figure 5a). The additions of biochar with 0.3 and 0.6 mass portions significantly increased the permeability coefficients of the concrete by 18.85% and 35.63%, respectively, in comparison to Control-ERC. The correlation between the addition of biochar and the permeability coefficients of the concrete indicate that this coefficient increases linearly as the amount of biochar incorporated increases within a specific range (Figure 5b).

3.2.2. Continuous Porosity

Closely related with the permeability performance, the continuous porosity of the concrete was also significantly increased by the admixture of the biochar in the ERCs (p < 0.05) (Table 1, Figure 6). However, the continuous porosity in the two biochar-integrated ERC groups did not show a significant difference. Generally, the porosity of pervious concrete is primarily controlled by the macropores formed between aggregates, which are largely determined by the particle size and distribution of the aggregates and have minimal direct correlation with additives such as cement and biochar [29]. Therefore, it is postulated that the porous structure of biochar may exert a certain influence on the porosity of pervious concrete, potentially enhancing its macroporous structure and thereby increasing the porosity of pervious concrete, but only to a certain extent.

3.2.3. Strength of the Concrete

There is no significant difference in the strengths among the three types of concrete (Table 1), which proves a consistent manufacturing process. Moreover, the concrete strength can be tailored to meet specific application purposes and life expectancy based on more realistic requirements.

3.3. Adsorption Performance of HM Ions from SWW with the ERCs

Before the filtration, the concentrations of the six HMs in the SWW were determined to be As 355.01 μg/L, Cd 79.51 μg/L, Cr 3262.70 μg/L, Mn 2353.99 μg/L, Pb 551.74 μg/L, and Cu 549.11 μg/L, respectively (Figure 7). The measured concentrations of most HMs in the simulated wastewater effluent were approximate to their nominal concentrations, but a difference was exhibited for some HMs because of the chemical reactions within the system. For example, Pb²⁺ precipitated with Cr₂O₅²⁻. After the filtration process, the concentrations of all these HMs were significantly decreased in the SWW and met the standard for irrigation water quality in China [30], except that of Cr (

Table 2. Heavy metal (HM) concentrations in the solution before filtration and after filtration through ERCs.

Samples	As (µg/L)	Cd (µg/L)	Cr (µg/L)	Mn (µg/L)	Cu (µg/L)	Pb (µg/L)
Simulated wastewater before filtration	355.01 ± 0.84	79.51 ± 0.67	3262.70 ± 20.78	2353.99 ± 3.41	549.11 ± 10.67	551.74 ± 41.15
Filtrates through Control-ERC	151.45 ± 17.86	64.23 ± 2.12	2320.91 ± 178.84	1649.52 ± 66.54	360.42 ± 9.09	467.72 ± 41.63
Filtrates through Biochar-ERC -1	32.893 ± 7.84	4.10 ± 0.81	2777.24 ± 123.983	157.62 ± 21.33	163.57 ± 7.55	156.85 ± 11.73
Filtrates through Biochar-ERC -2	24.935 ± 5.74	3.72 ± 0.05	2760.43 ± 95.54	132.41 ± 3.83	164.53 ± 1.21	200.89 ± 45.85
GB 5084-2021 [30]	50–100 a	10	100	none	500–1000 a	200

Note: ^a The standard concentrations depend on the crops.

). These results indicate that ERCs possess significant efficacy in absorbing most of these HMs from water. Chromium is extensively used in textile dyes and mordants, plating, pigments, alloying, etc., covering many walks of life [31]. However, removing Cr from wastewater is very challenging due to its rapid transformation between multiple oxidation states ranging from −2 to +6 [32]. This complexity is compounded by the fact that Cr can coexist in up to eleven different species in water [33]. Consequently, eliminating Cr from wastewater using a solitary technique is a formidable task. To date, various technologies have been developed to remove Cr from wastewater, encompassing physicochemical technology, electrochemical technology, and advanced oxidation technology. The method proposed in this study falls under the category of physicochemical technology, and it can involve multiple filtration stages or a filtration system to further reduce Cr concentrations and ensure compliance with the aforementioned standards.

Furthermore, the adsorption performance of HMs from the SWW using different ERCs was evaluated (Figure 8). Specifically, when the SWW was filtered through Biochar-ERC-1 and Biochar-ERC-2 concrete blocks, it resulted in the removal of more than 90% of As, Cd, and Mn, indicating that both treatments were exceptionally effective in eliminating these three HMs. The removal rates for all other HMs were greater than 60%. In contrast, filtering SWW through Control-ERC concrete removed less than 40% of all HMs, except for As, which was removed by about 60%. Notably, the removal rates of HMs except Cr were significantly lower for Control-ERC compared to the other two concretes containing shell-derived biochar. This suggests that shell-derived biochar exhibits strong adsorption capacity for HMs, which should be attributed to the increased porous structure and rich functional groups of the biochar. This postulation needs to be further verified. The lower removal rate of Cr with Biochar-ERC-1 and Biochar-ERC-2 may be attributed to the presence of Cr-repelling substances within the shell-derived biochar, which also needs to be further verified. In addition, Biochar-ERC-2 demonstrated higher adsorption capacity for HMs (except Pb) compared to Biochar-ERC-1. This suggests that excessive addition of shell-derived biochar can affect the adsorption of Pb to some extent. The removal of Pb in wastewater is not particularly increased with biochar addition, which may be due to the complex existence state of Pb ions in aqueous solution, the selectivity and efficiency limitations of biochar, as well as the influence of other factors in the wastewater [34]. Therefore, in practical applications, we need to integrate multiple treatment methods according to the specific composition and conditions of the wastewater to achieve the best removal efficacy.

Figure 9 illustrates the alterations in pH, DO, and conductivity observed in the filtration experiment. The use of concrete for filtration led to an increase in the effluent’s pH, while the incorporation of biochar into the concrete had a negligible impact on the effluent’s pH. Additionally, the application of concrete for filtration resulted in an elevation of effluent conductivity and a reduction in DO content. However, the introduction of biochar caused a decrease in both effluent conductivity and DO content.

In regions such as southern China, the southeast coast, and the middle and lower reaches of the Yangtze River, high precipitation leads to the easy leaching of alkaline substances from the soil, ultimately resulting in a predominantly acidic or strongly acidic soil pH level [35]. Concrete filtration can elevate the pH value of the water, making ERCs a promising material for use in these areas. By neutralizing soil acidity, it can improve the local soil environment, which helps optimize the growing conditions for crops and promotes healthy plant growth, ultimately boosting agricultural yield and quality. Overall, eco-recycled concrete with shell-derived biochar exhibits significant effects on the control of heavy metal pollution in water bodies, which is of positive significance for environmental protection and human health.

3.4. Limitations of the Preliminary Study

This preliminary study mainly focused on the permeability, porosity, and adsorption efficiency of ERCs. To further enhance its comprehensiveness, additional analyses such as the durability of ERC blocks, their long-term effectiveness, temperature resistance, and potential side effects should be incorporated, especially considering their practical application in different environmental conditions.

4. Conclusions

Shell-derived biochar and aggregate recycled from construction waste were shown to be cost-effective additives in concrete for improving its performance in multiple aspects. The biochar significantly increased the water permeability coefficients and the continuous porosity of the ERCs. It also significantly enhanced the adsorption efficiency of the concrete for removing HMs from wastewater. Furthermore, filtration with ERCs increased the pH and demonstrated a stable performance in maintaining the acid–base balance of water, which is particularly suitable for regions with acidic soils, such as southern China, the southeast coast, and the middle and lower reaches of the Yangtze River. Overall, the addition of shell-derived biochar greatly broadens the application potential of ERCs in the field of water treatment and ecological restoration. As technology advances and the scope of application expands, this innovative material is poised to play an increasingly important role in environmental protection and sustainable development.