In-Situ Remediation of Heavy Metal–Contaminated Sediments Using the Resuspension Technique

Abstract

Heavy metal pollution in sediments and soil is an unavoidable anthropogenic issue with implications for quality of life and is a major long-term remediation challenge. This paper aimed to evaluate an in-situ remediation technique (resuspension) for sediment that may be employed in a variety of contaminated site cleanup programs. Surface sediment samples were obtained from a shallow harbor on the St. Lawrence River, in Canada in 2019. Harbor sediment from the St. Lawrence River in Quebec is anthropogenically polluted by metals. Various experiments were performed using a designed reactor to evaluate sediment resuspension remediation technology. The method is based on sediments with a higher specific surface area that adsorb more metal contaminants. Therefore, the objective was to remove this fraction by the resuspension technique. Results showed that the levels of seven metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) were reduced by removing only 2.63% of the sediment. Removal efficiency values varied from 3.48% for Cd to 32.4% for Cu). The results of the sequential extraction tests imply that the resuspension technique is capable of decreasing the risk of remobilization of heavy metals in the aquatic ecosystem. Therefore, this method could potentially be used to remediate metal–contaminated sediment with minimal sediment removal.

1. Introduction

Contaminated sediment is a widespread problem with the potential to threaten the health and integrity of aquatic environments [1]. In the hydrological cycle, around 1% of pollutants are dissolved in water, whereas more than 99% are stored in sediments, which are the major sinks and carriers of contaminants in aquatic environments [2]. Therefore, knowledge of the sources and types of pollutants in sediments is essential [1].

The EPA has indicated that the decontamination of sediments will receive the highest priority. Characterization of the metal-contaminated sediments can assist environmental decision-makers in the management of the aquatic ecosystem [3]. Metals are among the most challenging pollution issues in the ecosystem because of their resistance to decomposition and subsequent bioaccumulation [4,5,6]. Metals are adsorbed and accumulate in sediments due to various mechanisms. As sediments are mobile, the contaminants can travel substantial distances from the pollutant sources [4,6].

Heavy metals usually possess significant toxicity to aquatic organisms and human health through bioaccumulation in the food chain. Hence, investigating the transformation and distribution mechanisms of heavy metals in sediment becomes necessary [7]. Large-scale studies on trace element contaminants in marine [8,9], coastal [10,11], and freshwater [12,13] sediments have been reported from various countries, indicating the large extent of the issue.

Sources of metals in sediments vary and include natural (weathering of soil and rock, erosion, forest fires, and volcanic eruptions) and anthropogenic sources (e.g., industrial effluents, urban wastes, mining and refining, agricultural drainage, domestic discharges, and atmospheric deposition), point and nonpoint sources, and spills [14,15]. The main source of heavy metals in the sediments may largely be the result of human activities, with the highest concentrations often measured in rivers, lakes, and reservoirs located in the cities and near industrial parks and towns. Sediments are the ultimate receptor of heavy metals, and the heavy metals sorbed from the water bodies of rivers, lakes, and bays eventually accumulate in the sediments [16].

The overall objective of heavy metal remediation is to minimize the risk of these toxic compounds on human and ecological health [17]. Although there is more information on technologies for the remediation of metal-contaminated soil, much less is known about sediment treatment. The properties of sediments including higher clay and organic matter contents can differ significantly from soils, and therefore, technologies that work for soils might not be as efficient for sediments [18]. The selection of an appropriate remediation technique depends on the characteristics of the site, the level of the metal contamination, and regulatory limits for the heavy metal(s) of concern in that regulatory domain. The remediation methods can be broadly divided into two major strategies: (1) in situ and (2) ex situ.

In-situ strategies focus on improving metal stabilization, which mainly occurs by enhancing metal sorption, precipitation, and complexation capacity of the sediment. Therefore, the potential mobility or bioavailability of the toxic metals to the environment will decrease [7]. In-situ techniques (such as capping, phytoremediation, resuspension) are logistically favorable, as they are of relatively low cost, less disruptive to the environment, and reduce the need for dredging, handling, or transportation of hazardous substances that generate the need for additional waste disposal [7,19,20].

In ex-situ remediation technologies, the contaminated material is removed from the site for subsequent treatment. This can either take place in an above-ground treatment facility (on-site) or by treatment or disposal elsewhere (off-site) [21]. In this strategy, polluted sediment is dredged from the bottom, and contaminants are extracted from the sediment through a series of chemical, physical, and biological methods in a specially designed reactor. Ex-situ sediment remediation is often the first choice for the heavily polluted sediments [7]. After dredging, various remediation approaches (such as electrochemical remediation, washing, and flotation) are available or under development [7].

Ex-situ remediation is the main viable option for some pollutants, and in-situ techniques are mainly used to reduce the mobility of the contaminants. On the other hand, dredging the contaminated sediment can increase the risk of mobility and availability of heavy metals in the harbors and impact the disposal sites that receive the dredged sediment [22].

The most popular in-situ approach, capping with or without reactive modifications, is not appropriate for the studied site because the primary issue is shallowness. By capping, the water depth is reduced, and the pollution remains at the location. Reducing water-sediment interactions and immobilizing the contaminants is the only benefit. Furthermore, in certain harbors with fine-grained silt, sand capping is ineffective because the sand layer may be loosened, and the contamination may seep through [23,24]. Thus, it would be ideal to create new methods for handling polluted sediment that are more adaptable and cause the least amount of environmental damage. It is important to note that both organic and inorganic contaminants are typically present in harbor areas. As a result, the remediation method should be suitable for both organic and inorganic pollutants at the same time [22].

The resuspension technique is based on the concept of the sediment particle–specific surface area and adsorption theory. Finer sediments have a larger specific surface area (i.e., clay and silt), so they have a greater tendency to adsorb the contaminants [22,23,24]. One of the benefits of this method is that the aeration in the water column not only suspends the sediments but also creates an aerobic condition in the lower layers of sediment. Additionally, its primary function is to stop the creation of hydrogen sulfide and eutrophication [23]. The resuspension’s ability to be used simultaneously for the cleanup of both organic and inorganic pollution is another benefit. Furthermore, this method does not require any chemical reagents. Compared to the large volumes of contaminated sediment recovered by dredging, much lower amounts are removed by this method, reducing transportation and disposal issues [22].

Resuspension was originally used as a remediation method for contaminated sediments used in Fukuyama City Port by Fukue et al. [23] to enhance the water-sediment quality and slow down the rate of eutrophication. They created a pilot project to selectively remove the sediments in the Fukuyama Canal Port. Instead of using the dredging procedure to remove all of the sediment at the canal’s bottom, they only removed 3% of it. According to their findings, the resuspension successfully decreased the chemical oxygen demand (COD), total phosphorus (T-P), and total nitrogen (T-N) by 31%, 14%, and 27.6%, respectively. However, they suggested that T-N, T-P, and COD may be reduced by roughly 10% by eliminating resuspended sediment. Pourabadehei and Mulligan demonstrated the feasibility of reducing the level of contamination of sediment through removal of finer sediment (clay and silt) that has more tendency to adsorb the contaminants [22].

The objectives of this study are to (1) determine the level of metal contamination in the sediments of the harbor, (2) assess the extent of contamination of the heavy metals using sediment quality guidelines and enrichment factors, (3) determine the influence of the resuspension technique on metal remediation and the relationship between metals and particle size of the sediment, (4) evaluate the effectiveness of this method for reduction of metal mobility from the contaminated sediment, and (5) assess risk reduction of sediment samples by the resuspension technique.

2. Materials and Methods

2.1. Site Characteristics

The study area is located at a yacht club in the St. Lawrence River in the province of Quebec in Canada. The area of the harbor is around 15,000 m². One solid and two floating breakwaters are present to protect the harbor from the waves. The water depth ranges from 0.6 m (near the dock area and shore) to 3 m (toward the outside of the harbor) [22].

2.2. Sediment Sampling

The sampling locations were chosen according to the previous research on pollution sources of the yacht club [25]. The study sites of the present investigation were sampled during September 2019. To evaluate the spatial variation, eight sampling sites were selected in 2019 for each sampling campaign. Duplicates of surface water and sediment were obtained at the locations in Figure 1. A Birge–Ekman Tall SS grab sampler (Wilco Co., Buffalo, NY, USA) was used for sampling sediments and to target recent deposition layers. It is versatile, durable, and ideal for soft bottoms, as found at the site. The Birge–Ekman grab sampler is the single most commonly used device for sampling upper sediments in limnic environments [26,27,28]. The area of the sample taken was 15 cm × 15 cm, and the height of the sampler was 22.5 cm. The samples were sealed in plastic bags, cooled to 4 °C in a portable cooler, transported to the laboratory, and stored in a 4 °C refrigerator for future analysis.

All containers were new or treated with 2.5% (v/v) hydrochloric acid and 5% (v/v) nitric acid (trace metal grade) for a minimum of 8 h followed by rinsing twice with Milli-Q 18µΩ cm prepared deionized water. To ensure quality control, blanks (deionized water) and controls were analyzed in duplicate.

Table 1. Sources of heavy metal contamination in the harbor.

Antifouling paints (heavy metals, i.e., Cu, Zn, Pb, and Cd)

Sewage

Wastewater

Petroleum released from motorboats

shows the sources of metal contamination in the harbor [22].

2.3. Experimental Setup

The setup for resuspension of the sediment (Figure 2) consisted of the aeration section with an air jet linked to the laboratory’s central compressed air system and a vertical cylinder of 20 cm in internal diameter and 50 cm in height. The cylinder sidewall features multiple outlet holes positioned at varying heights: hole 1 (1.5 cm), hole 2 (6.5 cm), hole 3 (11.5 cm), hole 4 (16 cm), hole 5 (21 cm), hole 6 (26 cm), hole 7 (31 cm), hole 8 (35.5 cm), hole 9 (40.5 cm) (Figure 3). Sediment samples were homogenized and subsequently combined with tap water at a ratio of 1:10 (v/v) in the reactor, reaching a height of 40 cm. Tap water was utilized in all experiments as the river was the source of tap water. The plastic cover avoided the loss of water and sediments during aeration. After 2 h, aeration was stopped to allow the settling of the coarser sediments. After 15 min, the coarse silt and sand particle fractions almost completely settled. From hole 6, the slurry in the reactor that included water, suspended particulate matter (SPM), and organic matter was extracted and directed to the filter system.

When designing sediment decontamination using the resuspension technique, it is essential to consider the connection between the parameters of resuspension in the setup and the use of this method. A sequence of lab-scale experiments and evaluation of sediment characteristics is required to identify the best resuspension parameters for managing contaminated sediments.

The resuspension method (regarding the duration of aeration, position of jet, and airflow) relates to the particular experimental arrangement utilized in the present research. If an alternative configuration is utilized in the lab or when working in the field, adjustments to the aeration duration, jet location, and air flow are necessary for achieving the best outcomes.

A general takeaway from existing observations and the results of Pourabadehei [22] is that this method is applicable to very fine sediments and is site-specific. The elements influencing the resuspension method mentioned earlier indicate that it is crucial to consider the settling time when formulating the optimal approach to target and extract the effective sediment particles to reach the desired metal concentration post-treatment. Consequently, additional research is required on reduced resuspension time and varying percentages of SPM removal.

2.4. Filter System

Three filter layers of needle-punch nonwoven geotextiles (POAXL 001 0.537M (21¼”)) were used with an air permeability of 20 cfm (based on the Texel technical dataTexel Technical Materials, St. Elzéar, QC, Canada sheet) (Figure 4). The apparent opening size of the filter was 29.30 µm.

2.5. Analytical Parameters

2.5.1. Grain Size

The sediment particle size analysis was conducted with a laser scattering analyzer (HORIBA, Horiba Canada Inc., Burlington, ON, Canada, LA-950V2). The outcomes were represented as D50 (where 50% of the particles are smaller than that size) along with the proportions of sand, silt, and clay in the sample.

2.5.2. Loss on Ignition

Loss on ignition (LOI) is a prevalent and frequently utilized technique to determine the organic and carbonate composition of sediments [29,30]. The LOI was calculated using the following equation:

LOI = 100 × (DW105 – DW550)/DW105

where LOI represents the percentage of weight loss on ignition at 550 °C, DW550 indicates the sample weight following exposure to 550 °C (for 4 h), and DW105 denotes the weight of sediment samples dried in a 105 °C oven (16 to 24 h).

2.5.3. Monitoring and Control

The variation of dissolved oxygen (DO), pH, and oxidation–reduction potential (ORP) was measured during the resuspension test. The pH was determined with an OAKTON pH meter, while DO will be tracked using the OAKTON PD-650 meter. ORP MilliporeSigma Canada, Oakville, ON, Canada was measured with the OAKTON ORP Tester 50.MilliporeSigma Canada, Oakville, ON, Canada

2.5.4. Sediment Digestion and Analyses

Total concentrations of As, Cd, Cr, Cu, Ni, Pb, and Zn in the sediment were determined after acid digestion employing the EPA 3050B method [22]. This procedure involved digesting 1 g of the dry sample with repeated additions of 5 mL of nitric acid (70%—trace metal grade) and 9 mL of hydrogen peroxide (30%), along with HCl (35%) at the end digestion [22]. Metal concentrations were then determined using an elemental analysis performed by ICP-MS with a quadruple mass analyzer (Agilent Model 7700x Agilent Technologies, Mississauga, ON, Canada).

2.5.5. Sequential Extraction Method

Sequential extraction analyses were conducted to determine the fractionation of the contaminants present in the sediment samples. This method utilized is based on the work of Yong et al. [31] according to the five fractions: (1) exchangeable, (2) bound to carbonates, (3) bound to Fe–Mn oxides, (4) bound to organic matter, and (5) residual [32].

2.5.6. Total Suspended Solids (TSS)

The total suspended solids were determined using the standard method ASTM 2540-D. A [33] specified volume of sample was filtered through a pre-weighed and pre-dried (105 °C for 16 h) glass microfiber filter utilizing the vacuum filtration technique.

2.5.7. Quality Assurance

Certified reagent-grade chemicals were used in the experiments. All plastic- and glassware used was new or soaked in 5% (v/v) nitric acid and 2.5% (v/v) hydrochloric acid (trace metal grade) for at least 8 h followed by two rinses with deionized water prepared using a Milli-Q 18 µΩ cm. Analytical quality assurance was conducted through the use of controls, blanks, and duplicates. In this study, the detection limits of the metals were 0.007, 0.002, 0.03, 0.1, 0.01, 0.002, and 0.009 ppb for Cr, Ni, Cu, Zn, As, Cd, and Pb, respectively. To calculate the recovery efficiencies, the matrix reference material EnviroMat contaminated soil was used. The analytical procedure for the recovery test was the same as that for field samples. The recoveries were 104.7 on average for above heavy metals.

2.6. Removal Efficiency

The removal efficiency of the resuspension technique was determined from Equation (1):

Removal efficiency (%) = (concentration before the treatment-concentration after treatment)/(concentration before treatment) × 100

(1)

All the calculations and analyses were conducted using Excel (version 2412).

3. Results and Discussion

3.1. Quality of the Sediment Before Resuspension

Table 2. Sediment sample analyses results in 2019 (mg/kg) (the bold and highlighted numbers represent the element concentrations that surpass the occasional effect levels (OEL) and probably effect levels (PEL), respectively).

St. No	Cr	Ni	Cu	Zn	As	Cd	Pb
1	55.91	29.24	450.30	243.61	7.21	0.75	63.59
3	61.12	32.60	255.92	248.87	8.59	0.75	56.39
4	66.94	35.61	164.41	304.32	7.68	0.99	49.30
6	41.64	28.16	34.82	138.78	5.49	0.47	22.32
8	68.59	38.11	118.47	247.20	9.93	0.77	59.45
9	78.10	40.80	126.14	229.22	8.65	0.88	54.54
11	69.23	35.75	66.61	223.84	6.65	0.75	31.12
14	74.72	42.39	127.05	215.42	15.52	0.77	32.77
Postglacial Clay	150	75	54	150	8	0.2	16
OEL	57	47	63	170	8	1.7	52
PEL	90	-	200	110	17	3.5	91
FEL	120	-	700	770	23	12	150

shows the level of metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) in the sediment for the year 2019. Among the various stations, the bold numbers indicate element concentrations that surpass the OEL (occasional effect level). Concentrations above the FEL were not detected. Some metals such as Cr, Cu, and Zn exceeded the guideline values for most sites. Consequently, the sediment samples may be classified as contaminated sediment that requires management. In addition, regular monitoring and further health risk evaluations are essential for metal levels that exceed the limits moving forward. Typically, stations 1, 2, 3, and 4 were the most polluted sites (near the maintenance area on the shore) as anticipated. These stations receive the runoff from the shore and primarily contain the elements found in antifouling paint. Cu, Cr, and Zn are commonly found in many antifouling paint formulations [34].

Extensive use of antifouling paints has released a significant amount of pollutants into the aquatic ecosystem via runoff into rivers, which accumulated in the sediments. The antifouling paint particles (APPs) result from boat repainting and repair activities. APPs contain inorganic and non-degradable biocidal components and other toxic substances, including copper and tributyltin (TBT) [35]. As triorganotin formulations have recently been banned, Cu (I)-based biocidal pigments and zinc oxide are now used for marine antifouling paints [36]. Various additives and non-biocidal pigments have been comprised of cadmium yellow, lead antimonates, and lead chromates more recently [37].

3.2. Exceeding Index (EI)

This approach adjusts the recorded metal concentration concerning a baseline level and is frequently utilized as a marker for pollution. As per the definition, when the concentration of substances surpasses the baseline level noted before industrial activities and is sufficiently elevated to negatively impact benthic organisms, it can be referred to as contamination [22]. Findings from earlier studies by Pourabadehei [22] indicated that the contaminant levels and particle size distribution of the sediments bear a closer resemblance to postglacial clays found in the St. Lawrence River than to pre-industrial sediments. These clays usually were deposited earlier in a marine environment. Consequently, the natural concentrations of metals in postglacial clays were used as a baseline level (Table 2) [22].

The Exceeding Index (EI) is characterized by the enrichment factor definition utilized by [38,39]. The EI represents the ratio of the total concentration of each metal to its natural concentration in postglacial clays. If the EI value is 2 or lower, it suggests that the metal originates from crustal sources in the sediment primarily through weathering or riverbank erosion. Conversely, an EI value exceeding 2 may be viewed as the input of activities of biota and/or human activities. Figure 5 illustrates the EI for eight chosen metals found in the sediment samples. According to the EI findings, Cd, Cu, and Pb are the primary elements that surpass their natural levels. Cd also displayed a high EI for all stations even though its concentration was below the OEL.

3.3. Physical Characterization of Sediment Samples Before the Resuspension

Table 3. Physical characteristics of sediment samples (2019).

St. No	LOI%	D50 (µm)	Clay (%)	Silt (%)	Sand (%)	Pebble (%)
1	9.18	325.33	1.36	19.02	71.68	11.91
3	14.27	279.00	1.22	19.74	75.56	5.23
4	14.90	63.39	5.06	49.57	45.22	0.22
6	5.90	51.52	10.50	50.32	37.78	4.20
8	11.06	25.80	12.20	66.38	21.41	ND
9	15.64	73.56	7.44	46.91	45.65	ND
11	10.76	14.68	17.50	67.20	15.30	ND
14	9.30	7.12	47.61	42.88	9.51	ND

Note: ND not detected.

shows a summary of the physical data of the sediments. The sediments consisted primarily of sand and silt particles with a small clay fraction. The sediment samples collected from stations 1 and 3 (2019), near the shore, exhibited coarser textures compared to the other sediment samples (with 71%, 75%, and 54% sand, respectively). The sediment organic matter content, represented as loss on ignition (LOI), ranges from 5% to 15% across the stations.

3.4. Quality of the River Water Samples

The metal concentrations in the river water samples collected were determined.

Table 4. Dissolved concentrations of aqueous metals (μg/L) at the site (2019) in comparison to Canadian Water Quality Guidelines (CWQG) for freshwater rivers [22].

	Cr	Ni	Cu	Zn	As	Cd	Pb
Average river water (2019) (Mean± S.D.) (n = 10)	2.31 ± 3.15	1.22 ± 0.94	6.82 ± 4.43	18.95 ± 2.88	10.81 ± 4.98	0.04 ± 0.05	0.81 ± 0.43
CWQG (CTC 1)	(11–16) 2	65	(variable) 3	30	5	0.8	2

Notes: ¹ Chronic Toxicity Criterion. ² National Recommended Water Quality Criteria. ³ The CWQG for Cu is related to water hardness (as CaCO₃ ), which ranges between 2 and 4 μg/L.

presents the findings of the dissolved metals in river water samples along with the standard levels in freshwater rivers as recommended by Canadian Water Quality Guidelines (CWQG) [22]. Cu was detected at levels higher than the allowable limits established by the chronic toxicity criterion (CTC). As per the EPA standard (criterion maximum concentrations; CMCs) [22], the average acute soluble Cu concentrations in freshwater varied from 2.37 μg/L for the most vulnerable species to over 107 mg/L for those that are of the lowest sensitivity [22]. Thus, evaluating the toxicity of this concentration of dissolved copper in river water depends on the species present at the location [25]. The levels of the other metals were within acceptable limits.

3.5. Physicochemical Characteristics of Sediment Samples During Resuspension Experiments

Laboratory analyses were carried out to assess the pH, dissolved oxygen (DO), and oxidation–reduction potential (ORP) values. The DO and ORP showed that the surface sediments were oxic prior to the test. Figure 6 illustrates the changes in DO, ORP, and pH throughout the resuspension tests. While resuspending the surface sediment from 2019, the DO concentration of the slurry rose quickly in the initial 10 min and continued to increase as the resuspension period concluded. Nonetheless, the DO gradually diminished and returned to the original level after 2 h. ORP rose significantly during the resuspension. The improvement of ORP and DO was due to the air injection during the resuspension [40].

The average pH for all samples during the test exceeded 5. Furthermore, it experienced a slight increase following air injection in every experiment. The rise in water pH aligns with several earlier studies [40]. Among the many influencing factors, pH is a primary element, and its impact on the fractionation of metals is crucial for the migration and transformation of those metals.

3.6. Physical Characteristics of Sediment and SPM Samples After the Test

Particle size distribution of the sediment samples and the suspended particulate matter (SPM- the particles remaining suspended for 15 min) was determined after resuspension) (

Table 5. Sediment samples and SPM particle size distribution after the test (2019).

	Sediment-After the Test						SPM
St. No.	LOI%	D50 (µm)	Clay (%)	Silt (%)	Sand (%)	Pebble (%)	LOI%	D50 (µm)	Clay (%)	Silt (%)	Sand (%)
1	11.70	96.05	9.85	33.77	47.01	9.37	9.69	5.28	44.67	52.22	3.11
3	11.67	42.41	12.63	41.92	41.65	5.70	13.54	1.36	61.57	37.89	0.54
4	18.56	78.76	6.28	41.11	47.00	11.24	13.04	9.66	7.14	91.80	1.09
6	5.98	31.32	10.01	47.10	42.67	0.22	9.87	4.56	96.95	3.05	--
8	15.10	51.81	12.76	38.96	47.39	2.67	11.37	0.24	72.04	25.91	2.05
9	14.29	26.37	5.45	58.75	35.54	0.39	13.42	0.17	99.61	0.39	--
11	11.55	28.67	25.01	40.88	32.74	4.12	10.71	0.28	62.70	35.91	1.38
14	11.55	7.19	36.13	41.89	21.83	0.48	9.99	0.23	81.15	18.78	0.21

). Estimating the quantity of sediment removed during the resuspension test is crucial. The effectiveness of resuspension can be greatly influenced by both the quality and quantity of SPM that is removed. This research aimed to eliminate smaller quantities of sediment that had higher contamination levels compared to earlier studies [22].

The particle size distribution indicated that the average D50 was approximately 104 microns prior to testing was quite fine. The average sediment particle size of the post-test was 45 microns. The sediments following the test were mainly composed of silt and sand. SPMs constituted the portion of the initial sediment that was suspended during the experiment and subsequently removed [22]. In this research, clay size was the prevalent particle size identified in SPMs.

Table 6. Metal concentration in sediments after the resuspension test (mg/kg) (2019). (The bold and highlighted numbers are the concentrations of elements exceeding the OEL and PEL, respectively).

St. No	Cr	Ni	Cu	Zn	As	Cd	Pb
1	43.49	24.23	252.99	200.39	5.36	0.63	45.93
3	60.37	32.49	137.60	233.38	7.78	0.70	55.78
4	66.70	34.57	137.52	329.45	5.76	1.12	51.47
6	31.82	20.38	31.44	113.00	3.52	0.37	15.89
8	53.50	34.83	55.02	229.40	5.59	0.90	30.96
9	77.31	41.06	103.13	255.89	7.15	0.80	35.43
11	57.35	34.58	47.89	199.40	5.24	0.74	28.34
14	69.15	42.55	72.48	229.72	7.15	0.76	36.53

shows the overall concentrations of metals found in the sediment samples following the 2-h resuspension. For various stations, the bold and highlighted values represent the concentrations of metals that surpass the OEL and PEL, respectively. The Okfindings showed that the concentrations of metals in all samples were reduced overall. Before the test, there were five metals with concentrations exceeding the OEL in 7 stations, which decreased to four metals for the same seven stations. In one case, copper in station 3, was reduced to below the OEL from the PEL.

The overall SPM levels of metals were also measured (

Table 7. Total metal contents in SPMs (mg/kg) (2019) (the bold and highlighted numbers are the concentrations of elements exceeding the OEL and PEL, respectively).

St. No	Cr	Ni	Cu	Zn	As	Cd	Pb
1	84.28	52.37	301.54	358.18	6.08	0.81	48.58
3	78.32	78.32	209.02	314.93	6.95	0.42	47.12
4	76.69	44.81	195.73	316.60	4.43	0.92	37.58
6	79.65	47.93	164.95	288.44	4.48	0.42	29.91
8	77.35	47.50	140.84	370.65	5.34	0.47	31.12
9	102.72	56.74	163.77	346.39	7.92	0.70	35.92
11	82.48	82.48	158.94	311.91	5.13	0.49	29.03
14	80.58	47.18	141.54	298.42	4.95	0.45	31.99

). The SPM size distribution ranged from 0.17 to 9.66 microns (clays and silt) for all sediment samples. SPMs exhibited higher levels of contamination compared to the bulk sediment samples, aligning well with earlier research [22]. The Enrichment factor (EF) serves as an index for comparing metal concentrations in SPMs and sediments following the test [22]. Hence, for metals that have an EF value exceeding one, the process of SPM resuspension in aquatic environments proved effective in decreasing the contamination. Removing additional SPM will result in a reduction of contaminants left in the environment in the remaining sediments [22].

Table 8. Enrichment factor values for metals in sediments (2019).

St. No	Cr	Ni	Cu	Zn	As	Cd	Pb
1	1.94	2.16	1.19	1.79	1.13	1.30	1.06
3	1.30	2.41	1.52	1.35	0.89	0.59	0.84
4	1.15	1.30	1.42	0.96	0.77	0.82	0.73
6	2.50	2.35	5.25	2.55	1.27	1.16	1.88
8	1.45	1.36	2.56	1.62	0.96	0.52	1.00
9	1.33	1.38	1.59	1.35	1.11	0.87	1.01
11	1.44	2.38	3.32	1.56	0.98	0.67	1.02
14	1.17	1.11	1.95	1.30	0.69	0.60	0.88
Mean	1.53	1.81	2.35	1.56	0.98	0.82	1.05

shows the EF values for metals in sediment samples and indicates that the EF values were in the following order: Cu > Ni > Zn > Cr > Pb > As > Cd > (2019), with average EF values exceeding one. Nevertheless, the EF values for As and Cd were markedly lower, suggesting that the resuspension process might not be very efficient in removing As and Cd at certain stations.

3.7. Removal Efficiency (RE)

Table 9. Removal of metals (%) (±SE) by resuspension (2019). ND indicates not significant removal.

St. No	Removed Sediment (%)	Cr	Ni	Cu	Zn	As	Cd	Pb
1	2.92	22.22	17.12	43.82	17.74	25.59	15.71	27.76
3	2.37	1.24	0.34	46.23	6.22	9.38	6.70	1.10
4	2.48	0.36	2.93	16.35	ND	25.02	ND	ND
6	1.58	23.58	27.61	9.71	18.58	35.86	22.99	28.78
8	2.28	22.01	8.62	53.56	7.20	43.74	ND	47.92
9	3.54	1.00	ND	18.24	ND	17.35	8.45	35.04
11	2.21	17.16	3.27	28.11	10.92	21.14	1.99	8.93
14	3.65	7.46	ND	42.95	ND	53.97	2.04	ND
Mean	2.63 ± 0.25	11.8 ± 3.6	7.3 ± 3.5	32.3 ± 5.7	4.2 ± 4.1	29.01 ± 5.1	3.4 ± 4.7	16.7 ± 7.4

indicates the RE values for sediment samples from 2019. In general, RE values averaged positively for all metals in 2019, ranging from a low of 3.48% for Cd to a high of 32.4% for Cu. Consequently, the resuspension technique effectively lowered the concentration of metals (As, Cd, Cr, Cu, Ni, Pb, and Zn) by extracting only 2.63% (finer suspended solids contain higher concentrations of heavy metals) of the polluted sediment. Additionally, this technique is more flexible for managing contaminated sediment and causes minimal harm to the surrounding environment. Cr, Cu, and As showed positive RE values across all sediment samples. Furthermore, according to the findings presented in Figure 5, Cr, Zn, Ni, As, and Cu (with the exception of stations 1, 3, and 5) were below EI of 2 and were deemed not contaminated. RE values may not be significant for Cu, As, Ni, and Cr, which indicated concentrations at or near background levels.

3.8. Sequential Extraction Test Analysis (SET)

It is crucial to examine various types of metal mobility and bioavailability instead of just the total concentration to gain insight into the bioavailability of metals. Therefore, sequential extraction methods are often utilized as they yield insights into the distribution of metals across various lattices within the solid sample, which effectively balances the information regarding potential environmental contamination risks [41].

Stations 1, 3, and 4, known for having the highest levels of contaminated sediment samples near the maintenance area, along with station number 14, which exhibited the smallest particle size, were chosen for the SET. Figure 7 illustrates the stepwise extraction of trace metals at stations 1 and 14. Sediment samples prior to the tests and SPM following the resuspension test were analyzed according to the five fractions. It is important to note that the effectiveness of the resuspension method heavily relies on the removal of metals via SPM. Consequently, determining the metal fractionation in SPM is essential for this research.

Metals in the exchangeable fraction F1 are the least tightly bound in the sediments, making them more readily bioavailable. The greater the concentration of metals in this fraction, the more easily they are mobilized, increasing the potential threat to the environment [42]. As per the findings of SET, 44% of Cd was found in the exchangeable fraction at station 3, peaking at 66% in station 14 within the bulk sediment. It is widely recognized that metals from human activities that enter water bodies via industrial and household discharges are more readily remobilized compared to metals that occur naturally in the environment [42]. The research conducted by Huang et al. [42] revealed that Cd constituted 54% of the exchangeable fraction. At stations 4 and 14, the presence (F1 + F2) of Cd in SPM was slightly lower than that in the initial (pre-suspension) sediments. Cd levels found in the initial sediment samples were greater, which may be harmful to benthic organisms and could accumulate in these organisms [43].

At stations 3, 4, and 14, the availability (F1 + F2) of Pb in SPM was marginally greater than in the initial sediments. Therefore, in those stations, the resuspension method effectively eliminated some of the Pb with elevated concentrations in more accessible fractions via SPM removal. The Pb and Cd concentrations in the more stable fractions of SPM were slightly lower than those in the initial sediment samples.

Fraction F3 for stations 1, 4, and 14 in SPM were higher than the initial sediment for Cd and Pb. This may be due to the presence of Fe/Mn oxides, which are excellent scavengers for trace metals [44]. They can remobilize in the aqueous phase and can scavenge metals through a combination of adsorption, coprecipitation, and surface complex mechanisms.

As shown in Figure 7, the copper concentration percentages in F3 were greater in SPM than in the initial sediment samples. In addition, copper was primarily (approximately 84%) in the more stable organic and residual fractions within the initial sediment. Nonetheless, in SPM, copper levels were discovered to be more prevalent in available fractions, particularly in F3, on average. This conduct was similarly noted by additional research [22,45]. Furthermore, resuspension may also promote desorption and the release of loosely bound metals into the water column. Conversely, metal fractionation displayed significant variation due to alterations in physical and chemical factors like redox potential and pH [46,47]. Zheng et al. [40] demonstrated that pH is a crucial factor affecting the liberation of metal from sediment. Reduced pH levels were linked to an increased desorption of metals from sediment. Increased pH levels were observed to improve metal adsorption through produced oxides or hydroxide precipitation. As the pH rose slightly during resuspension and subsequently remained above 7.0, the scavenging of metals may primarily be enhanced by the presence of iron and manganese oxides. Earlier research has indicated that the increased aqueous oxygen concentration with an ORP > 80 mV, Fe (II) and Mn (II) may undergo oxidation to form Fe (III) and Mn (IV), potentially leading to precipitation [22,48]. Soluble metals can adsorb on these precipitates [22,48]. Another investigation into the resuspension of polluted sediment under varying pH conditions showed that the release of metals from the sediments to the overlaying water was linked to a pH decrease [49]. This might clarify why the F3 fraction in SPM played a more significant role than in the initial sediment. In other words, the metals released from the sediments during the resuspension to the water column may likely adsorb or coprecipitate with Fe–Mn oxides due to the high specific surface area of suspended particulate matter (SPM). The SPM was subsequently removed.

Zinc behaved differently. In stations 3 and 14, the percentage of zinc concentrations in F3 was higher in the initial sediment than in SPM. Furthermore, zinc was found to be less available (F1 + F2) in the SPM in comparison to the initial sediment in stations 1 and 4. F3 is the most important fraction for Zn in both the initial sediment and SPM. The findings align with earlier research, which showed that Fe and Mn oxide/hydroxides serve as significant binding sites for Zn [50].

3.9. Effect of Resuspension on the Water Quality

Evaluating the water quality after the contaminated sediment resuspension is crucial. A significant drawback of remediation methods such as dredging is the potential for leaching of metals into the water phase [51]. Therefore, if the release of metals into the water falls within the acceptable limits, the resuspension process may be a viable remediation method. The Canadian Water Quality Guidelines, CWQG [22], and national suggested water quality standards [22] were utilized for the evaluation. For instance, findings from research conducted by Nayar et al. [52] showed that for every metal, higher levels were found in the suspended particulates compared to the sediments. Dissolved metals were low and in some cases were undetectable. Dissolved trace metals in water may be complicated by inorganics or organics and occur as free-hydrated ions. In studies of fractionation, differentiating between the ‘inorganic’ and ‘inert’ metal fractions is particularly significant. The inorganic fraction comprises free forms and complexes noted for rapid dissociation rates, whereas the inert fraction signifies the tightly bound metal complexes. The inorganic component would reflect the bioavailability and harmfulness of metals [53].

The release of contaminants in every sample of this study was indicated by the concentration of dissolved metals in the water after the resuspension test.

Table 10. Aqueous metal concentrations after resuspension test and comparison to river and tap water and reference standards to assess water quality (μg/L).

	Cr	Ni	Cu	Zn	As	Cd	Pb
Concentration Ave. (n = 16)	0.27 ± 0.26	0.69 ± 0.20	3.28 ± 1.64	6.54 ± 9.21	0.51 ± 0.18	0.02 ± 0.01	0.11 ± 0.28
Min.	0.03	0.42	0	0	0.49	0.00	0
Max.	0.94	1.26	6.81	35.52	1.01	0.06	1.89
Ave. River water (n = 14)	1.31 ± 2.57	0.88 ± 0.78	4.81 ± 3.61	10.02 ± 6.57	6.47 ± 6.37	0.02 ± 0.04	0.46 ± 0.50
Ave. Tap water (n = 10)	0.06 ± 0.06	1.05 ± 0.19	461.88 ± 40.52	16.56 ± 7.39	0.56 ± 0.06	0.005 ± 0.004	0.68 ± 0.60
CWQG	N/D	(25–150)	(variable 1)	30	5	1	(1–7)
CCC 2 (Chronic)	11 3	52	1.45	120	150	0.25	2.5
CMC 4 (Acute)	16 3	470	2.37–107,860 5	120	340	2	65

Notes: ¹ The CWQG for Cu is related to water hardness (as CaCO₃) normally in the ranges of 2 to 4 μg/L. ² Criterion continuous concentrations [22]. ³ Recommended values for chromium VI. ⁴ Criterion maximum concentrations [22]. ⁵ Species mean acute values (SMAVs) ranged from 2.37 μg/L for the most sensitive species ‘Daphnia pulicaria’, to 107,860 μg/L for the least sensitive species ‘Notemigonus crysoleucas’ [22].

presents the mean values of metal concentrations along with the reference standards and the identified dissolved metals in both tap water and river samples.

Overall, the CWQG imposes stricter regulations compared to the EPA’s standards. Nevertheless, the different standard values present a broader perspective on the short- and long-term impacts of pollutants on aquatic organisms and provide additional possibilities for comparison [22]. The findings indicated that the levels of all metals in the surface water were below the CWQG.

The levels of metals were lower than the criterion continuous concentration (CCC or chronic effect level) in both standards, whereas Cu exceeded the chronic effect level for certain sensitive species. Cu is the sole metal that demonstrated concentrations exceeding the CCC and CMC for the most sensitive species (Table 10). These findings were shown to be in strong alignment with literature values regarding water quality following sediment resuspension [22]. In a different study carried out by Hwang et al. [52], water quality data was assessed following resuspension in two varieties of river sediments (anoxic or oxic). The findings suggested that a resuspension event might cause short-term water quality decline in both sediment conditions.

Copper exhibits a broad spectrum of acute values because the criterion maximum concentration (CMC) of copper varies according to the sensitivity of various species [54]. Nevertheless, the level of dissolved copper is considerably elevated compared to the CCC shown in Table 10. This is mainly due to the existence of organic matter based on additional research [55,56], which showed that Cu is primarily found in the organic fraction. Furthermore, the typical concentration of Cu in river water samples is inherently elevated (e.g., exceeding 4.81 μg/L).

The pH has a significant influence on the mobility of metals in sediment. Typically, as the pH in the sediment drops, the solubility of metals increases. It then reduces the adsorption capacity and subsequently enhances the mobility of metals. Occasionally, just a slight decrease in pH units can reduce the retention of metals on sediment particles from nearly total to none [7]. In this research, the pH rose during resuspension, inhibiting the remobilization of metals in the water phase. Additionally, the time spent on resuspension was another aspect, which was relatively brief in these experiments. For instance, it has been noted that, even in sulfidic sediment, minimal quantities of metals are discharged into the aqueous phase during a brief resuspension lasting 6 h at a pressure of five dynes per square centimeter [57].

3.10. Evaluating the Efficiency of the Filtration System

The extracted slurry generated by the resuspension was subsequently directed to the filtration system. This experiment was replicated for six different samples six times. The initial geotextile average hydraulic flux is 11.088 L/m²/h (measured as liters per filter area per hour (L/m²-h);

Table 11. Mean particle size of SPM before and after filtration.

St No.	Mean Particle Size of SPM Before Filtration (µm)	Mean Particle Size of SPM After Filtration (µm)
St 1	9.97	1.14
St 3	3.62	0.69
St 4	11.70	2.10

). The apparent opening size of the filter is 29.30 µm. The filters were effective for decreasing the mean size of particles. Table 11 presents the characteristics of the slurry after filtration. The particle size decreased from 8.43 to 1.31 µm. The concentration of TSS in the slurry was determined before and after filtration (

Table 12. TSS before and after filtration and hydraulic flux of three layers of filter.

St No.	TSS Before Filtration (g /L)	TSS After Filtration (g /L)	TSS Reduction (%)	Hydraulic Flux (L/m2/h)
St 9	4.40	0.093	97.87	4.36
St 6	2.56	0.117	95.41	12.9

). The mean TSS values reduced from 4.40 g/L to 0.093 g/L. Therefore, the stated filters proved adequate for filtration objectives, taking into account the sediment nature based on findings from this experiment. The Canadian Council of Ministers of the Environment [33] states that the permissible level of TSS is 0.025 g/L for surface water. The suspension was sent to the filtration system. However, the water criteria were not satisfied after the filtration, so additional processing is required. Optimization of the filtration step will be performed in future research.

4. Conclusions

As sediment pollution by metals is a global issue, efficient remediation methods are needed. To tackle this issue, environmental managers need effective, scientific methods to assess the possible effects of sediment-bound chemicals on different resource applications (e.g., aquatic organisms or wildlife that consume aquatic organisms). This study has specifically concentrated on the problem of harbors characterized by a combination of shallow depths and sediment contamination due to metals. As dredging sediment leads to a rise in oxygen and turbidity levels, which heightens the risk of metal mobilization into the water, a different management approach to dredging for polluted sediment is necessary to decrease contaminant levels and the potential risk to the aquatic ecosystem.

The sediment quality was evaluated using the Canadian sediment quality guidelines. The overall levels of Cr, Cu, and Zn exceeded the recommended limits, indicating that the sediment is contaminated with metals and could potentially have negative impacts on this region. The exceeding level (EL) indicated that sediments examined in this research were significantly contaminated by Cu, Cd, Pb, and Zn while being moderately contaminated by other metals. The resuspension technique effectively lowered the levels of 7 specific metals (As, Cd, Cr, Cu, Pb, Ni, and Zn) by eliminating only 2.63% of the polluted sediment. Average removal efficiency values were beneficial and ranged from 3.48% for Cd to 32.4% for Cu. The impact of the resuspension method on the fractionation of metals in sediment and SPM fractions was assessed. The results of the sequential extraction indicated that SPM with elevated EF values had higher contaminant concentrations in less stable fractions. Hence, the risk of metal mobilization might be decreased after the resuspension technique and removal from the environment of this SPM. The water quality following treatment was also evaluated. The metal contents (As, Cd, Cr, Ni, Pb, and Zn) were lower than the criterion continuous concentration (CCC or chronic effect level) in both standards, while Cu was the only metal that exceeded the chronic effect level for certain sensitive species. This indicates that the resuspension is a low risk to the water environment.

This study was performed as a laboratory-scale experiment, although the sediment samples used were from the site (St. Lawrence River). Scaling up the equipment for pilot tests is recommended. Moreover, performing the resuspension technique on different types of sediments with different size distributions and various contamination levels is recommended.