Flash-Calcined Sediments for Zinc Adsorption

Abstract

Heavy metal contamination of the environment could pose health risks to humans. Until recently, several geochemical routes were considered to address the issue of metallic leachates from sources such as e-waste deposal sites or mining areas. Following the adsorption pathway, this study focused on investigating the ability of calcination products as a low-cost option for the remediation of zinc contaminated effluents. Sediments dredged in northern France were calcined by flash calcination method, before testing the calcination products (FCS₇₅₀ hereafter) as adsorbent for zinc removal. The calcination process performed at 750 °C resulted in a denser material with a higher specific surface area and lower organic matter content, suited for aqueous remediation. Kinetic and equilibrium assessment underlined a rapid adsorption better described with the Pseudo Second Order model than the Pseudo First Order or Bangham equations. Adsorption models (Langmuir, Freundlich and Temkin) implemented to describe the interaction under two pH conditions (pH = 2; pH = 7) resulted in a maximum adsorption capacity (q_max) of 89.61 mg g⁻¹ under the most favourable configuration. The pH conditions had poor influence on the adsorbing capacity value, which suggested a good buffering property of the calcined sediment and a potential applicability on effluents with different chemistries. A q_max comparison based on 42 studies indicated that FCS₇₅₀ was more beneficial than most raw organic adsorbents but remained less effective than grafted and composite materials. Nevertheless, the low number of steps necessary for FCS₇₅₀ processing, the easy access of its precursor (harboring wastes) and its low energy cost suggested good economic competitiveness and pleaded in favor of field experimentations.

1. Introduction

In most industrialized countries, the management of metal leachates from waste disposal sites is a serious environmental problem, especially near electronic waste repositories undergoing meteoric weathering issues [1,2]. The toxicity to receiving aquatic systems is now a well-described phenomenon, with exacerbation episodes when leachates mobilize a mixture of heavy elements instead of a single type [3]. Toxicity also intensifies when the metallic ions interact with other polluting agents, such as micro plastics, that may originate from the same pool [4]. Zinc is a very communal pollutant in the environment and can influence the water’s ecological environment [5]. At high concentrations in the soil, zinc becomes phytotoxic, and zinc accumulation through absorption or deposition poses health risks to consumers [6]. To prevent this, a large panel of remediating methods with varying outcomes are now explored in the literature. The spectrum of solutions covers principles such as coagulation, membrane filtration or electrodialysis. Each of these routes has advantages such as dehydration in the case of coagulation, small space requirements in the case of filtration techniques and high metal segregation in the case of electrodialysis. However, they face severe cost barriers [7] and supplementary drawbacks, such as excessive sludge production [8], which hinder field implementation strategies. Various biological techniques relying on the heterotrophic regime of microbial strains [9,10,11] are also being investigated to provide sustainable solutions. Most microbial approaches underline the primordial roles of hydraulic retention time [12,13] and chemical oxygen demand [14,15] in treatment devices, while others emphasize the importance of the nature of organic substrates implemented as electron donors [16,17].

Among the remediation options, methods based on the adsorbing properties of inorganic substrates also show promising results [18,19]. The extensive bibliography devoted to this geochemical route emphasizes the prominent role of the intrinsic properties of contaminated systems (organic matter content, grain size distribution), environmental parameters (pH/Eh) and the design of experimental setups. Removal efficiency is also assessed in terms of adsorbent properties, such as specific areas and cation exchange capacity. The operation is generally performed by means of phyllosilicates, especially smectites [20] and kaolinites [21,22]. The mechanism consists of replacing an exchangeable cation by the problematic metallic ion under specific kinetic and thermodynamic conditions. Background parameters (pH and temperature) play a major role by constraining the intensity of these exchanges. The ideal pH range varies according to the mass and valence of the involved cation. For Zn, most studies refer to pH magnitudes ranging from 6 to 8 [23]. These values should, however, be taken with caution, as other removal mechanisms, i.e., precipitation under oxide and hydroxide forms, prevail under basic conditions. It is, therefore, difficult to distinguish removal via adsorption stricto sensu from removal through precipitation phenomena with the experimental designs adopted in most of these studies.

This work falls within the scope of adsorptive methods and is intended to evaluate the contribution of calcined sedimentary wastes for zinc removal. Indeed, waterways maintenance generates important amounts of dredging materials each year, with increasingly expensive storage costs for managing entities and public authorities [24]. Several recycling pathways are already being investigated to transform this type of waste into eco-responsible components for civil engineering [25,26] and thus facilitate their incorporation in circular economy schemes. In recent years, calcination treatment has emerged as a useful preliminary step in valorization processes. Snellings et al. [27], for instance, proposed a thorough characterization (physicochemical and pozzolanic activity) of fine-grained sediments before assessing the behaviour of cement blends [28] made with calcination products. The recycling of calcination products is also considered during the conception of new types of concrete [29,30]. This valorization option faces, nevertheless, notable mechanical consistency issues [31] that hinder field application considerations. On the other hand, calcined sediments display interesting magnitudes for specific areas [32], which may confer a valuable behaviour regarding the issue of heavy metals’ mobility. Although a wide spectrum of recycling possibilities is being considered in civil engineering, there are no experimental approaches attempting to upgrade calcined sediments as adsorption substrates to our knowledge. Therefore, we explore here their potential as stabilizing materials to propose an alternative pathway for their incorporation within circular economy schemes.

Most adsorbents implemented for similar purposes are inorganic substrates with high specific areas, or activated carbons deriving from a lignocellulosic precursor [33,34]. To meet these criteria, precursor sediments were collected from the river channel of Noyelles-Sous-Lens (Northern France), known for its fine and highly organic detrital content [32], before flash calcination. The physicochemical properties (mineralogy, density, surface area, environmental behaviour) of the Flash-Calcined Sediment (FCS₇₅₀) and its precursor were characterized and batch experiments were subsequently conducted with FCS₇₅₀ to determine the ideal Solid/Liquid (S/L) ratios for Zn removal in aqueous conditions. Additional sets of experiments were undertaken with the objective of characterizing the kinetic of interaction and to determine the equilibrium under different background conditions (pH, salinity). In a second step, FCS₇₅₀ was blended with Zn-doped raw sediments to test its potential for the remediation of solid organo-mineral matrices.

2. Materials and Methods

2.1. Sediments and Calcination Method

Raw sediments were collected from the fluvial depot of Noyelles-Sous-Lens (NSL) in the Hauts-de-France region (Northern France). Manual homogenization and quartering were performed before drying (105 °C), grinding (<80 µm) and flash-calcining the material, following the method reported by San Nicolas et al. [35,36]. The technique aims to produce metastable mineral phases [37] and is usually applied for clay activation [36,38]. It is known for increasing the surface defects of crystallites, augmenting the number of reactive sites [39], and eliminating organic matter, as well as anthropogenic pollutants. The flash calcination was carried out in a flash furnace in the research center of IMT Nord-Europe (France) at 750 °C. This temperature was selected as it allows the activation of the material, especially in terms of its pozzolanic properties [32], with a minimum of energy consumption and noxious off-gases release.

2.2. Zinc Removal Experiments

2.2.1. Aqueous Zn/Calcined Sediments: Batch Experiments

Optimal calcined Sediment/Liquid (S/L) ratios by mass were assessed before determining isotherms under different conditions of pH and salinity. An analytical reagent grade product ZnCl₂ from Fisher Scientific^® (Illkirch, France) was dissolved at 15 mmol L⁻¹ in deionized water before the experiments. The tests were performed in 8 vials (100 mL) for which the S/L ratios were, respectively, 0.01, 0.013, 0.02, 0.04, 0.1, 0.2, 0.25 and 0.5. These values were retained to test the effect of FCS₇₅₀ on a wide interval of S/L ratio. After 24 h of FCS₇₅₀/Zn interactions under continuous stirring (100 rpm), aliquots of 10 mL were sampled, filtered (0.45 µm) and acidified (HNO₃) for Zn quantification via Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES). The experiments were replicated in their entirety.

In this part, two types of bath experiments (kinetics and isotherms) were carried out. Kinetics assessments were performed at relatively high volumes (1 L) to minimize S/L ratio variations. After FCS₇₅₀ addition (100 g) into 1000 mL adsorbate with an initial concentration of 15 mmol L⁻¹, 10 mL of aqueous samples were taken at each step of the following time schedule: 0.5 min, 1.5 min, 2.5 min, 5 min, 10 min, 30 min, 60 min, 120 min, 240 min, and 480 min, with an initial S/L ratio of 0.1 and under 100 rpm of agitation. The aliquots were immediately filtered (0.45 µm), acidified (HNO₃) and kept cold at 4 °C before ICP-OES quantifications.

The interactions between FCS₇₅₀ and Zn²⁺ under growing adsorbate concentrations was performed at 0.1 of S/L ratio. The adsorbate concentrations were 0.15 mmol L⁻¹, 1.50 mmol L⁻¹, 7.64 mmol L⁻¹, 15.29 mmol L⁻¹, 38.20 mmol L⁻¹, 76.40 mmol L⁻¹; 114.66 mmol L⁻¹ and 152. 88 mmol L⁻¹, respectively. The pH_ini was adjusted by adding a variable amount of analytical grade of HCl (0.1 M) or NaOH 0.1 M, while the salinity was adjusted using an analytical grade of NaCl dissolved in deionized water. Aliquots of 10 mL were sampled after 24 h of stirring (100 rpm) and kept cold after filtration and acidification.

2.2.2. Stabilization of Zn-Spiked Sediments Using the Calcined Sediment

Spiking Experiments

In addition to the aqueous tests, the efficacity of FCS₇₅₀ on polluted organo-mineral matrices has been assessed via a doping method. Dredging operations conducted in the rivers of La Deule (Lille, Haut-de-France, France), Aa (Saint-Omer, Haut-de-France, France) and La-Lys (Aire sur La Lys, Haut-de-France, France) between November 2019 and February 2020 provided three different types of sediments. Each sediment was dried (45 °C) until constant mass, then finely ground before the spiking experiments. The spiking consisted in adding 10 g of sediment in 100 mL of deionized water with different concentrations of Zn²⁺ (76, 38, 15, 7, 0.15 and 0.015 mmol L⁻¹). To monitor Zn²⁺ concentration absorbed by these sediments, each set of sediment was stirred (100 rpm) for 24 h, at which point 10 mL of aliquots were sampled, filtered (0.45 µm) and acidified (HNO₃) before measurement in ICP-OES.

Spiked Sediments’ Stabilization Tests

Sediments spiked at an equal level (15 mmol L⁻¹) were dried at 45 °C (7 days), blended with an increasing amount of FCS₇₅₀ homogenized (10 min, 100 rpm), and completed with 100 mL of ultrapure water with the objective of evaluating the contribution of FCS₇₅₀ as a stabilizer. For each spiked matrix, FCS₇₅₀ was added over a range between 0.2 and 32%. The mixtures were stirred at 100 rpm (24 h), and aliquots of 10 mL were sampled for chemical analysis (ICP-OES).

2.3. Kinetics and Sorption Modelling

2.3.1. Kinetic and Reliability

Pseudo First order (PFO), Pseudo Second order (PSO) and Bangham equations were implemented to describe the temporal evolution of Zn removal. As introduced by Lagergren in the late 19th century, the PFO equation can be summarised with the following expression (1):

$$\log \left(qe-qt\right)=logqe-\left(\frac{k_1}{2.303}\right)t$$

(1)

where q_t is the amount of adsorbed ions (mmol g⁻¹) at a given time (t); q_e represents the amount of the adsorbate at equilibrium (mmol g⁻¹), and k₁ (min⁻¹) is the rate of adsorption.

The PSO which is generally used for a long period of adsorbent/sorbate interaction can be expressed as follows, proposed by Ho et al. [40]:

$$\frac{t}{qt}=\frac{1}{k2qe}+\frac{1}{qe} t$$

(2)

where q_t and q_e are the same parameters in the PFO equation; and k₂ (g mmol⁻¹ h⁻¹) is the PSO rate of adsorption.

The Bangham equation assumes fast velocity of adsorption [41] and can be written as follows (3):

$$\log qt=\log kb+\frac{1}{n}\log t$$

(3)

where k_b is the Bangham sorption rate constant.

Consistency of the Kinetic Models

In addition to the R², chi-square test (χ²) and Average Relative Error (ARE) were used to check the consistency of PFO, PSO and Bangham models with the experimental data. The equations of χ² and ARE can be written as follows:

χ² = Σ (q_{e exp} − q_mod)²/q_{e exp}

(4)

ARE = 100/n Σ (q_{e exp} − q_mod)/q_{e exp}

(5)

Here, q_{e exp} is the amount of adsorbed Zn at equilibrium (mmol L⁻¹), here determined after ICP OES quantification, q_mod (mmol L⁻¹) the equivalent of this amount inferred from the kinetic models and n the number of measurement points during the kinetic experiment.

2.3.2. Adsorption Isotherm Models

Three equations (Langmuir, Freundlich, Temkin) commonly used to model the sorption of ionic compounds onto mineral surfaces were implemented to characterize the relation of FCS₇₅₀ with Zn under two experimental conditions (pH = 2; pH = 7). The Langmuir model assumes adsorption occurring at individualized sites hosting a unique ionic compound. The idea was postulated of modeling gas molecules’ adsorption onto plane surfaces [42] and was further generalized to other types of ions and surfaces. Its linear expression can be written as follows:

C_e/q_e = q_max K_L/[1 + (K_L C_e)]

(6)

with C_e the concentration of Zn²⁺ in the solution at equilibrium (mmol L⁻¹), q_max the maximum sorption capacity of the adsorbent (FCS₇₅₀) and K_L the Langmuir constant (L mmol⁻¹).

The Freundlich model takes into consideration surface heterogeneities. Its linear equation can be expressed as follows:

log q_e = log K_F + 1/n (log C_e)

(7)

where K_F (g L⁻¹) and n are constants and indicate, respectively, the extent of the adsorption and the degree of non-linearity between the sorbate and the adsorbent. When the term 1/n is between 0 and 1, the adsorption mechanism is favored [43].

The Temkin model assumes a linear decrease of the heat of adsorption with coverage due to adsorbent–adsorbate interactions [44]. It can be summarized as follows:

q_e = B (ln K_T + ln C_e)

(8)

where B is the isotherm constant (J mmol⁻¹) and K_T the equilibrium binding constant (L mmol⁻¹), respectively

2.4. Characterization of Adsorbent

Multi-techniques approaches were employed to determine the properties of materials before and after flash calcination, including the chemical-mineralogical composition, transformation of clay phases, particle size distribution, specific density and specific surface area.

X-ray fluorescence (XRF): The chemical compositions of FCS₇₅₀ and its raw precursor in terms of oxides were measured using a Bruker S4 Pioneer^® spectrometer and a 4-kW wavelength dispersive X-ray fluorescence spectrometer equipped with a rhodium anode. Analyses were conducted at 60 kV and 40 mA on fused beads made from 10 g of melting agent and 2 g of sedimentary material. An integrated standardless evaluation was conducted to realize a semi-quantification down to the part per million level.

Particle size distribution, specific density, surface specific area and organic matter content: LS12330 (ISO 13320-1) laser diffraction Coulter was used to determine particle size distribution within FCS₇₅₀ and its precursor with the isopropanol used as the dispersant. Specific surface areas were inferred via the Brunauer-Emmett-Teller (BET) method using a 3FLEX Surface Characterization Analyzer device from Micromeritics. A Helium Pycnometer (Micromeritics ACCUPYC 1330) was used to determine density values. The organic matter content of materials was measured according to the XP P94-047 standard.

X-ray diffraction (XRD): A Bruker D2 Advance diffractometer system via Cu-Kα radiation and a fast Lynx Eye position sensitive detector (λ = 1.5406 Å) were used to identify the mineralogical composition of dry powdery samples (<40 µm). The diffractometer was set at 40 kV and 40 mA. Scans were undertaken from 5° to 80°, with a step interval of 0.02° and an acquisition time of 96 s per step. To assess the transformation of clay minerals in materials before and after flash calcination, the fine fraction of materials (particle size smaller than 2 µm) was analysed with a Bruker D8 Advance device equipped with a Co-Kα radiation (λ = 1.789 Å) set at an acquisition range from 5° to 35° and a step size reduced to 0.008°.

Thermogravimetric Analysis: To observe the transformation of materials as function of temperature, a Thermogravimetric Analysis (TGA) was performed on the powder material sample using a Netzsch STA 409 device under Argon flux with a heating rate of 10 °C/min and a temperature range from 40 °C to 1000 °C.

Sediments leaching: The mobility of metallic trace elements and anionic elements was determined after leaching experiments according to the standard EN 1257-2, with a Solid/Liquid ratio by mass of 0.1 and an equilibrium time of 24 h. For this analysis, 100 g of sediments (Dmax < 0.4 mm) was introduced into 1000 mL of ultrapure water, and shaken for 24 h. Then, leachates were filtered at 0.45 µm, and acidified with 2% ratio by volume HNO₃ commercial product (63% concentration by mass). An ICP-OES 5100 from Agilent Technologies^® was used to analyze metallic elements, and anionic species were quantified using a Dionex^® ICS-3000 DC Ionic Chromatograph equipped with an AS 11 HC column (4 mm, 250 mm) associated to an AS-DV Dionex^® autosampler. Results were compared to inert waste (IW) and non-hazardous waste (NHW) thresholds as specified in the European Directive 1999/31/EC.

3. Results and Discussion

3.1. Physico-Chemical Properties of Sediments before and after Flash Calcination

The physico-chemical properties of sediments before and after flash calcination were determined in a preliminary step to assess the effect of thermal treatment. The results are given in

Table 1. Chemical composition (oxides) and physical properties of sediments before and after flash calcination.

		Before Calcination (Raw Sediment)	After Calcination (FCS750)
Chemical composition (% wt)	CaO	14.65	13.05
	SiO2	54.92	57.86
	Al2O3	13.36	13.97
	Fe2O3	7.10	6.20
	SO3	0.30	0.21
	Na2O	0.96	0.86
	K2O	2.55	2.62
	MgO	1.22	1.30
	ZnO	0.37	0.36
	P2O5	2.91	2.63
	Total	99.17	99.49
	LOI (950 °C)	27.63	13.7
Physical properties	Density (g cm−3)	2.43	2.63
	BET surface (m2 g−1)	4 38	15 59
	Organic matter content (%)	16.1	1.93
	d10 (µm)	1.24	2.63
	d50 (µm)	11.68	14.71
	d90 (µm)	55.91	64.13

LOI: Loss on ignition.

.

The amount of organic matter (OM) decreased from 16.1% in the raw sediment to 1.93% in FCS₇₅₀, which represented a loss of 88% of the OM initially disseminated within the sedimentary matrix. As a combined effect of OM, carbonates and volatiles loss, a denser material (2.63 g cm⁻³) with higher distribution of fine particles was obtained in accordance with previous results [38]. From a granulometric point of view, the magnitudes of d₁₀, d₅₀, and d₉₀ increased from 1.24, 11.68 and 55.91 µm in the raw sediment to 2.63, 14.71 and 64.13 µm, respectively, within the calcined product. Several authors have suggested that mechanical frictions desegregate coarse elements during the flash mechanism, thereby increasing the proportion of fine particles [28,45]. This increase of fine particle volume led very plausibly to surface area augmentation, with BET surface values rising from 4.38 m² g⁻¹ in the raw sediment to 15.59 m² g⁻¹ within FCS₇₅₀. The augmentation of the specific area may also have arisen from the increase in surface roughness consecutive to the loss of thermo-sensitive components (OM and carbonates). Composition in oxide was quasi similar in FCS₇₅₀ compared to its detrital precursor, although silicates (SiO₂) underwent a slight increase within the calcined sediment.

The mineralogical composition of materials before and after flash-calcination is presented in Figure 1A. The main mineralogical phases of raw sediment consist of quartz, calcite, albite, and clay phases, such as illite/muscovite, and kaolinite. These compositions were characteristic of fluvial sediments resulting from watershed erosions and in situ precipitation. Observing the XRD pattern of FCS₇₅₀ sediment, it can be seen that the flash calcination led to several changes in the mineralogical phases. Anhydrite was likely to have been derived from gypseous precursors transformed during the dehydration steps inherent to our analytical protocol. It also may have resulted from the recombination of CaO (calcite decomposition) with SO₄²⁻ from sulphated phases [46]. The shapes and intensity of quartz peaks on the diffractogram remained identical after calcination, emphasizing the absence of thermal transformation for these crystals. The peaks of the other silicates (albite) have been refined due to the vaporization of interfering constituents, namely organic compounds. The peak of calcite appeared less intense as a consequence of the thermal denaturation of this mineral. Calcite’s propensity to destructure under high temperatures by transforming CO₃ into CO₂ is, in fact, a well-known phenomenon [46]. Otherwise, FCS₇₅₀ exhibited peaks at 17° an 38.5° of 2θ which were not able to be spotted on the diffractogram of the raw sediment and not clearly indexed in identification tables. These peaks were very likely due to the diffractive expression of neo formed phases and recrystallisation phenomena.

Complementary characterizations, performed on oriented samples to determine the nature of clayey components within the raw sediment, as well as the transformation of these components after flash calcination, are shown in Figure 1B. The result was shown in a diffractogram with well refined peaks at 24°, 31°, 35° of angular values, corresponding to illite ((K, H₃O) (Al, Mg, Fe)₂ (Si, Al)₄ O₁₀), and two other intense peaks at 13.5°and 34° of 2θ, fitting to the diffraction pattern of kaolinite (Al₂ Si₂ O₅ (OH)₄). Illite remained unaffected after calcination, whereas a substantial loss of intensity affected the two peaks of kaolinite (13.5°, 34°). As kaolinite dehydroxylates at 600 °C [45], its lack of signature on the FCS₇₅₀ diffractogram was probably induced by its thermal destabilization during flash heating.

TGA data are presented in Figure 2 over the thermal window (50; 1000) and a DTG scale from 0 down to −0.35% min⁻¹. For the raw sediment, mass losses over the interval (50; 400) can be assigned to water evaporation and OM depletion. The inflexions on the intervals (400; 600) and (600; 800) can be attributed to kaolinite dehydrolyxation into metakaolin [47] and calcite decarbonation into CaO [32], respectively. From 50 to 800 °C, the DTG evolutions of the raw sediment and FCS₇₅₀ were synchronous, with differences only in terms of magnitude. The variations of FCS₇₅₀ were of lesser extent due to a lower occurrence of heat-sensitive components content. At 850 °C, a significant DTG loss, observed only in the data for FCS_750, was assigned to the thermo-gravimetric behavior of neo-formed phases.

The metallic trace elements and anionic elements of materials before and after flash calcination are given in

Table 2. Metallic and anionic species before and after calcination (leaching data). IW (Inert Waste) and (Non-Hazardous Wastes) according to the European directive 1999/31/EC.

		Before Calcination	After Calcination	IW	NHW
	As	<0.08	<0.1	0.5	2
	Ba	1.4912	1.9	20	100
	Cd	<0.008	<0.008	0.04	1
	Cr	<0.03	0.37	0.5	10
Trace metals (mg kg−1)	Cu	2.0509	0.078	2	50
	Mo	0.5538	1.7	0.5	10
	Pb	0.0447	0.074	0.4	10
	Ni	0.4799	<0.03	0.5	10
	Sb	-	<0.05	0.06	0.7
	Se	0.1019	0.08	0.1	0.5
	Zn	7.8518	<0.05	4	50
Anions (mg kg−1)	F	<1	8	10	150
	Cl	262	247	800	15,000
	SO42−	15,800	11,120	1000	20,000

. Regarding the leaching behaviour of the sediment, standardized experiments emphasized high mobilizations of metallic trace elements in the raw sediment. After calcination, the release of trace metals was below the acceptability limits (European Directive 1999/31/EC), indicating a beneficial effect of the thermal process. In addition, it can be seen that the flash calcination significantly reduced the content of sulfate in the calcined product.

3.2. Aqueous Interactions of FCS₇₅₀ with Zn

3.2.1. Adsorbent/Adsorbate Ratios and Interaction Kinetics

To design an experiment that would allow the deduction of a relevant q_max value, a preliminary set of experiments was conducted with the objective of determining an optimal Adsorbent/Adsorbate ratio or Solid/Liquid Ratio and the equilibrium time. FCS₇₅₀ progressive addition to solutions with constant Zn²⁺ concentrations (15 mmol L⁻¹) showed a plateau of adsorption at a S/L ratio of ~0.1 (Figure 3). This ratio was therefore retained for subsequent investigations. Kinetic experiments were in fact performed at 0.1 of S/L ratio to determine the duration necessary for equilibrium and to infer kinetic constants.

The results of kinetic experiments and models are summarized in Figure 4 and

Table 3. Kinetic parameters deduced from the Pseudo First Order (PFO), the Pseudo Second Order (PSO) and Bangham equations.

Kinetic Models	Associated Parameters	Values for FCS750
	qe exp (µmol g−1)	13.8
	k1 (min−1)	22.83 × 10−3
	R2	0.947
Pseudo First Order	qe mod (µmol g−1)	14.97
	χ2	9.34
	ARE	91.55
	k2 (g mmol−1 min−1)	42.20 × 10−2
	R2	0.999
Pseudo Second Order	qe mod (µmol g−1)	15.28
	χ2	13.70
	ARE	98.9
	kb (g mmol−1 min−1)	15.24 × 10−3
	R2	0.957
Bangham	qe mod (µmol g−1)	15.28
	χ2	13.70
	ARE	98.9

, respectively. It can be seen that a good correlation was observed between the experimental result and the models. The amount of adsorbate at equilibrium (q_e) obtained by the experiment was 13.8 (µmol g⁻¹ sorbent), while this value was 14.97 (µmol g⁻¹) and 15.28 (µmol g⁻¹) corresponding to the PFO, PSO and Bangham models, respectively. However, the data adjustments to PFO, PSO and Bangham models showed that the PSO approach (Figure 4) best fitted the measurements, with high magnitude of χ² (15.28 × 10⁻²), ARE (13.70) and R² (0.999, cf. Table 3). The good fit of PSO is consistent with results found in the previous study. For instance, in the case of Kwon et al. [48] experimenting on Zn removal with scoria, correlation coefficients were higher with the PSO model (0.999) than the PFO (0.895). Similar observations have been reported by Ghasemi et al. [44], Yang et al. [49] and Krishnan et al. [50], who showed better correlations with the PSO model compared to PFO. In addition to the correlation coefficient obtained in our approach (R² = 0.999), the error analysis values (χ², ARE) and the bibliographical trend suggest that the values of q_{e mod} and k inferred through the PSO model are more significant than those deduced from the two other models (PFO and Bangham).

3.2.2. Adsorption Capacity

With a relevant S/L ratio (0.1) and an interaction time (24 h), sets of experiments were undertaken with different Zn concentrations to assess the maximum adsorption capacity of FCS₇₅₀. Adsorption data were subsequently fitted to Langmuir, Freundlich and Temkin models according to their linear expressions (Figure 5). Fitting parameters showed that the data were better described by the Langmuir model compared to the other two (

Table 4. Isotherm parameters (Langmuir, Freundlich and Temkin) for the interaction experiments conducted at pH = 7 and pH = 2.

Isotherms	Associated Parameters	Values for FCS750 at pH 7	Values for FCS750 at pH 2
	KL (L mmol−1)	13.51	13.48
	qmax (mmol g−1)	1.35	1.37
Langmuir	R2	0.998	0.999
	KF (mmol g−1)	1.18	1.08
Freundlich	n	1.06	1.18
	R2	0.948	0.985
	B (J mmol−1)	0.090	0.096
Temkin	KT	1.07	1.07
	R2	0.937	0.931

). Consequently, the Langmuir model was considered to infer the maximum adsorption capacity of FCS₇₅₀. For the experiments conducted at pH = 2, the maximum adsorption deducted from Langmuir was 1.37 mmol g⁻¹ (89.61 mg g⁻¹), while a maximum capacity of 1.35 mmol g⁻¹ (88.30 mg g⁻¹) was obtained at pH = 7. The closeness of these two values suggested a buffering effect of FCS₇₅₀ instead of a significant influence of the initial pH on the uptake process. Contrary to this observation, several studies suggested an increase of Zn stabilization at pH = 7 as a response to growing alkalinity [51]. This was not observed here, probably due to the impact of FCS₇₅₀ on the acido-basic configuration of the medium during the blending steps.

Regarding the influence of the activity of dissolved ions, results have shown poorer Zn uptake from ≈0.4 mol L⁻¹ to superior NaCl concentrations (Figure 6), suggesting an inhibitive effect of ionic strengths beyond this threshold value. This behaviour is coherent with those of other adsorbents, such as zeolites [23] and bentonites, which were reported as more efficient at low ionic strengths.

3.2.3. Removing Capacity of FCS₇₅₀ Compared to Other Sorbents

The ability of FCS₇₅₀ to remove Zn from aqueous medium was compared to the maximum adsorption capacities of 42 other adsorbents categorized into five groups (Figure 7) labelled: raw organic adsorbents, activated organic adsorbents, raw inorganic adsorbents, activated inorganic adsorbents and composites. The components of each group and the references of the original studies are available as supplementary information (Supplementary Table S1). FCS₇₅₀ had higher removing capacities than the groups of non-activated and activated organic adsorbents, which comprised materials as various as bio-chars, palm tree leaves and modified coir fibers. The gain in specific area and amount of finely sized particles after the calcination process (Table 2) can reasonably be evoked as a boosting factor for Zn sorption in the case of FCS₇₅₀. These two intrinsic properties are well acknowledged for governing the intensity of sorption mechanisms [52,53], in addition to the role of the chemistry of the aqueous medium. The effectiveness of FCS₇₅₀ compared to the two groups of organic sorbents might have arisen from these two properties. On the other hand, FCS₇₅₀ appeared less efficient than the groups of inorganics and composites consisting of adsorbents such as scoria, modified zeolites and graphenes. One plausible hypothesis is that this inferiority was driven by the fact that most inorganic adsorbents possess a high number of exchangeable cations and admit negatively charged edges, e.g., the hydroxylated edges of clay minerals. Electrical charges distributed in such ways contrast with the a priori inert surface of FCS₇₅₀ and may explain the gaps in efficiency.

3.3. FCS₇₅₀ Application on Spiked Solid Matrices

To evaluate the stabilizing capacity of FCS₇₅₀ on polluted solid matrices, three different fluvial sediments (La Deule, Aa and La Lys) were spiked with Zn solutions before treatment. The mechanisms of Zn-trapping and its promoting factors are described in a previous study [51]. This study indicated that the Aa sediments had the best Zn retention properties because of a higher alkalizing capacity. Aqueous media alkalinization was, in fact, suggested as a more significant factor than organic matter content and/or particle size for Zn retention.

For the three spiked sediments, the ratio between Zn concentration at equilibrium and the initial concentration of Zn is expressed in Figure 8 as a function of growing FCS₇₅₀ amounts. As an influence of their intrinsic phyco-chemical properties (supplementary information, Supplementary Table S2), the three sediments responded differently to the treatment with FCS₇₅₀. The Aa sediment had the best reaction to the treatment, with a diminution of almost 50% of mobilizable Zn at 5% of FCS₇₅₀. With this same proportion of FCS₇₅₀, the treatment was less successful on the other two sediments. Indeed, only ≈30% and 10% of mobilizable Zn were stabilized for La Deule and La Lys, respectively (Figure 8). Nevertheless, the treatment proved to be effective with increasing amounts of FCS₇₅₀. It led to the stabilization of more than 70% of mobile Zn for the three sediments at the threshold value of 15% of FCS₇₅₀.

4. Conclusions

In order to evaluate the potential use of flash calcined sediments as an efficient adsorbent, the sediment was treated by the flash calcination method (at 750 °C) and tested via a lab-scale approach to address the environmental issue of aqueous zinc. Based on the results of this study, the following conclusions can be drawn.

The thermal process significantly reduced the amount of organic matter, which made the resulting material eligible for aqueous remediation. The flash calcination also induced higher specific surface areas (BET surface) and a moderate increase of particle size predisposing the calcined sediment for better interactions with ionic species.

An optimal Adsorbent/Adsorbate ratio of 0.1 by mass was determined for the two types of batch sorption experiments (kinetic and equilibrium). The experiments showed rapid adsorption of FCS₇₅₀ with almost all Zn²⁺ ions adsorbed within 120 min. The amount of absorbate at equilibrium obtained by the experiment was relatively similar to these obtained from the kinetic models. However, the kinetic sorption data of FCS₇₅₀ showed the best fit using PSO model with higher values of R², χ², and ARE.

A maximum adsorption capacity of 1.37 mmol g⁻¹ (89.61 mg g⁻¹) was measured under the most favourable configuration. Small changes of the adsorption capacity between acidic (pH = 2) and neutral (pH = 7) conditions suggested buffering effects in aqueous medium, making the calcined sediment a good candidate for the remediation of acidic effluents, such as mines’ and e-wastes drainages. A comparison with a bibliographical sample of 42 zinc adsorbents showed that FCS₇₅₀ had better removing capabilities than groups constituted of exclusively organic substrates, but remained less efficient than groups of engineered composites. Nevertheless, the simplicity of the thermal manufacturing keeps FCS₇₅₀ competitive, as the pyrolytic process is easier to implement than the multiple steps required for the development of most composite materials. Its impact on aqueous Zn suggests that effluent remediation could be a viable recycling option for waterway wastes, having similar physico-chemical properties, and encouraging further testing using effluents with complex chemistry and pilot-scale devices.