Geochemical characterization of different phases of phosphate sludge waste according to their density in seawater

doi:10.21203/rs.3.rs-4987026/v1

Phosphate treatment at the Kpémé plant uses hydrocycloning with seawater, which produces phosphate sludge wastes discharged into the sea without any prior treatment. This waste is highly enriched in trace elements, and is the source of water and coastal sediment pollution. A study of the bulk density of the phases obtained shows that the apatite-rich phase is denser than the clay-rich phase. Clay-rich phases will be transported over longer distances than apatite-rich phases, in accordance with the sea environment conditions prevailing on the coast. The clay-rich phase has a higher degree of acidity and redox potential than the apatite-rich phase. These phosphate sludgy wastes and the various phases are enriched in Cr, Cd, Cu, Co, Zn, Ni and Hg compared with the earth's crust. On the other hand, the light phase is more enriched in trace elements than the apatite-rich phase. The light phase has a higher K_d for all elements than the apatite-rich phase, except for Cd. This makes the light phase much more toxic once each phase is resuspended and carried by tidal currents to sea.

Phosphate sludge waste

phases

density

transport

trace elements

Sedimentary natural phosphates have a highly complex chemical composition. In addition to minerals such as apatite, clays, iron oxides and organic matter, they contain numerous metallic elements, some of which are considered toxic and have a harmful impact on the environment [1–10]. The south-eastern part of Togo is rich in phosphate ores located in the coastal sedimentary basin (Hahotoé-Kpogamé). Studies carried out since the beginning of these deposits' exploitation have shown not only high levels of phosphoric anhydride (P₂O₅), but also of certain metals such as Cr, Zn, Cd, Cu, Ni, F, etc. [5, 11–15]. The processing of these phosphates into commercial grade ore takes place at the Kpémé plant near the beach. Impurities (clay exogangue) are removed by wet sieving and hydrocycloning.

These activities generate massive quantities of various wastes such as solid and liquid muddy wastes (~ 2.5 million tons per year) rich in toxic heavy metals that pollute coastal waters and sediments; consequently, they cause harmful effects on ecosystem and human health [14]. The environmental impacts caused by some of these heavy metals have already been studied by a number of authors [5, 11–14, 16–20]. Work has also been carried out on the treatment and recovery of these muddy wastes that pollute the coastal environment [19]. It is therefore important to characterize the different phases transported and the associated pollutants during the dispersion of this waste once discharged into the sea. As the density of these phosphate wastes is not homogeneous, this enables us to evaluate the impact that each phase may cause during transport. It is likewise crucial to know the impact caused by each liter of phosphate waste discharged into the marine environment. That's what this study aims to achieve.

The coastal region of Togo is located between latitudes 6° and 7° North. Phosphate-related industrial activity is located in the southeast of this region, on two sites. These are the mining site at Hahotoé-Kpogamé and the raw ore processing site at Kpémé near the beach. Kpémé is one of the areas most affected by this industrial activity. Our study is based on samples of phosphate mud waste collected at the phosphate processing plant. The location of the study area is shown in Fig. 1.

The experiments and analyses were carried out in the Geology laboratory of the Department of Earth Sciences at the University of Lomé. The different phases of the waste were separated using a mini hydrocyclone Galaxi–Tower provided by AWAS GmbH, which enabled the particles to be classified according to their density. The concentrations of phosphate-sludge waste discharged into the sea were reproduced. Artificial saline water at 33g/L prepared from NaCl (99.5% purity) was used during the study to simulate natural seawater conditions. Sludge waste was dissolved in seawater at a ratio of 85g:1L and the experiment was carried out at a flow rate of 4.5 m³/h. The experimental set-up is illustrated in Fig. 2.

Bulk density (ρ_s) was determined using the graduated-tube method. In a pre-weighed test tube (M₀), the solid is filled to 100 mL (V). The filled test tube is reweighed (M₁). The following relationship was used to determine the bulk density:

$$\:{\rho\:}_{s}(g/cm³)=\frac{\left({M}_{1}-{M}_{0}\right)}{V}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(1\right)$$

pH_H2O, pH_KCl, redox potential (E_h) and electrical conductivity (EC) were determined in a 1:2.5 ratio; for every 10 g of sample weighed, 25 mL of water (pH_H2O and EC) or 1M KCl (pH_KCl and E_h) were added, and the mixture stirred for 5 min. The mixture was then left to stand for at least 2 h before measurements of the various parameters were measured in the supernatant [14, 21].

At pH 7, the sum of the exchangeable cations (Mg²⁺, Ca²⁺, K⁺, Na⁺) was extracted by ammonium acetate. The exchangeable acidity (A) was extracted by 1 mol (KCl) L^− 1 solution and titrated by volumetry. The effective cation exchange capacity (ECEC) is then expressed according to the following relationship:

$$\:ECEC\:(meq/100g)=A+\sum\:\left(M{g}^{2+},\:C{a}^{2+},{\:K}^{+},N{a}^{+}\right)\:\:\:\:\:\:\:\:\:\:\:\:\:\left(2\right)\:$$

To determine the apparent partition coefficient (K_d), shaking experiments were carried out by mixing 10 g of phosphate sludge waste powder or phase (< 1 mm) and 100 ml of artificial seawater (33g/L) in a 1:10 ratio. The seawater was added to the 10g in 200ml polyethylene bottles, which were agitated for 24 hours using a mechanical shaker. The residual solution was separated from the solid material by filtration in 100 ml bottles containing 1 ml HNO₃ through 0.45 µm membrane filters [13, 22]. K_d values, corresponding to the ratio of the concentration in the particulate phase (C_sed) and the concentration in the dissolved phase (C_aq), were determined using the following equation:

$$\:{K}_{d}=\frac{{c}_{sed}}{{c}_{aq}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)$$

Loss on ignition (LOI) was assessed on the sample, which was first oven-dried (105°C) and gradually heated in an oxidizing medium to 1,000°C, then stabilized at this temperature for 4 h in a Nabertherm muffle furnace (max 1,100°C).

Sediment digestion was carried out using a DigiPREP Jr. graphite digestion block system, fitted with a DigiPREP touch-screen controller and a DigiPROBE. The method used for digestion is adapted from the US Environmental Protection Agency's [23] method for digestion of sediments, wastes and soils. Using a Bioevopeak balance accurate to 0.001g, the dried sample was weighed (0.5 g) directly into the polypropylene flat-bottomed DigiTUBE. After adding 10 ml HNO₃ 1:1 and homogenizing the mixture, the screw cap was turned slightly to close the DigiTUBE (not very tightly) and then placed in the graphite blocks. The mixture was refluxed at 95°C for 15 minutes. The tube was then removed from the block to cool. After cooling, 5 ml HNO₃ (68%) was added. The mixture was heated again to reflux at 95°C for 30 minutes. The mixture was then evaporated for 1 hour and removed from the block. After cooling to room temperature, 2 ml double-distilled water was added and 5 ml H₂O₂ (30%) was added dropwise while manually rotating the tube until the bubbles disappeared. The tube was returned to the block and heated to 95°C for 2 hours. Next, 10 ml HCl (37%) was added and refluxed at 95°C for 15 minutes. The digestate was filtered with a DigiFILTER (0.45µm) after cooling to room temperature, then the filtrate was collected in a 100 ml container with double-distilled water. After filtration, the product remaining on the filter consisted of the unattacked residue and silica from the silicates. The funnel and filter were placed on a rack and small fractions of lukewarm 2% sodium hydroxide were poured in, the filtrate collected in a 200 mL container for silica analysis.

To validate the analytical methods and repeatability of assay results, duplicates were introduced randomly into the batches, and two standard samples (NCS DC 87102, NCS DC 73311) and a blank were also prepared together under the same experimental conditions. Also, all analytical material was preconditioned in 10% supra-pure HNO₃. The recovery ratios for standard samples ranged from 93.36-106.04% and 95.48-104.19% respectively. Statistical processing was carried out using Microsoft Office Excel 2019 and OriginPro 2019b; and for mapping, QGIS 3.34.8 was used. The methods and equipment used for the analyses are recorded in Table 1 below.

Table 1

Equipment and methods for parameter analysis
Parameters	Methods	Equipment
pH, E_h	Electrometry (NF T 90 − 008; BS ISO 11271)	Multimeter, Sanxin SX 736 (pH/mV/Conductivity/DO)
EC	Conductimetry (NF EN 27888)	Multimeter, Sanxin SX 736 (pH/mV/Conductivity/DO)
SiO₂	Colorimetry (HACH Method 8185)	Spectrophotometer HACH DR3800
Al	Colorimetry (HACH Method 8012)
P₂O₅	Colorimetry (NFT90-023)
F	Colorimetry (HACH method 8029)
Ca, Mg	Atomic absorption spectrometry (NF EN ISO 7980)	Atomic Absorption Spectrometer (AAS) iCE 3000 Series THERMO SCIENTIFIC
K, Na	Atomic absorption spectrometry (NF T 90 − 020)
Cd	Atomic absorption spectrometry (NF EN ISO 5961)
Fe, Mn, Cr, Pb, Cu, Co, Zn, Ni	Atomic absorption spectrometry (FD T 90–112)
Hg	Cold vapor hydride (NF EN 1483)	AAS iCE 3000 Series + VP100

Physicochemical properties of phosphate sludge waste and its various phases

The phosphate-sludge mine waste is a heterogeneous mixture of particles ranging in size from micrometers to submicrometers. The physical properties of these wastes, such as grain size and density, are therefore important in sedimentation and transport processes. The classification of waste particles on the basis of their density is necessary. Other parameters such as redox conditions, pH, conductivity and cation exchange capacity control the chemistry of these phosphate mining wastes and their various phases. The values of the various parameters are given in Table 2.

The bulk density in these phosphate sludge phases varies between 1.17 and 1.27 g/cm³, while the clay-rich phases show a variation of 1.18–1.29 g/cm³. Apatite-rich phases, on the other hand, fluctuate between 1.28–1.41 g/cm³. This difference in density observed in the phases studied shows that the apatite-rich phases are denser than the clay-rich phases obtained during the experiment.

A study of the pH values of the different phases showed that the pH values of the phosphate slurries varied between 6.19–6.62 for pH_H2O and between 5.85–6.27 for pH_KCl. In apatite-rich phases, these values range from 6.22–7.22 for pH_H2O and from 5.82–6.36 for pH_KCl, and in clay-rich phases from 6.20–6.69 for pH_H2O and from 5.76–6.18 for pH_KCl. On average, acidity is higher in the clay-rich phase than in the apatite-rich phase.

The electrical conductivity of phosphate slurries ranged from 7.56–22.40 mS/cm. For apatite-rich phases, these values range from 0.74–32.10 mS/cm, and for clay-rich phases from 1.20–26.10 mS/cm. On average, electrical conductivity is higher in apatite-rich phases than in clay-rich phases.

Electrical conductivity values show a strong, significantly negative correlation with pH_H2O values in muddy phosphate wastes (a), apatite-rich phases (b) and clay-rich phases (c) (Fig. 3).

In the case of redox potential, phosphate wastes show varying values 59–79 mV. In apatite-rich phases, these values oscillate between 42 and 62 mV, while in clay-rich phases they vary between 44 and 67 mV. Clay-rich phases have a higher redox potential than apatite-rich phases.

Table 2

pH, electrical conductivity, redox potential, bulk density and effective cation exchange capacity values in phosphate sludge waste and different phases (pH unitless, EC in mS/cm, E_h in mV, ρ_s in g/cm³ and ECEC in meq/100g)
	pH _H2O	pH _KCl	EC	E _h	ρ_s	ECEC
			Phosphate sludge waste (N = 9)
Min	6.19	5.85	7.56	59.00	1.17	25.53
Max	6.62	6.27	22.40	79.00	1.27	34.75
Mean	6.42	6.10	13.90	68.89	1.22	28.38
			Apatite-rich phases (N = 9)
Min	6.22	5.82	0.74	42.00	1.28	21.68
Max	7.22	6.36	32.10	62.00	1.41	28.73
Mean	6.66	6.16	12.97	52.56	1.36	23.64
			Clay-rich phases (N = 9)
Min	6.20	5.76	1.20	44.00	1.18	20.11
Max	6.69	6.18	26.10	67.00	1.29	25.37
Mean	6.45	5.98	10.23	57.78	1.25	23.02

With regard to cation exchange capacity, phosphate-mud wastes recorded values varying between 25.53–34.75 meq/100g; in apatite-rich and clay-rich phases, values ranged from 21.68–28.73 meq/100g and 20.11–25.37 meq/100g respectively. Apatite-rich phases have a higher cation exchange capacity than clay-rich phases.

Geochemical composition

The contents of major elements in phosphate sludge and their phases are given in Table 3. Oxides such as SiO₂, CaO, Na₂O, K₂O, P₂O₅ and F are used to describe the chemical composition of phosphates (Gnandi 1998). Some of the oxides analyzed are part of the apatite structure (CaO, P₂O₅ and F), while others are associated with the sedimentary phase and are either of detrital origin (K₂O) or the result of weathering (Fe₂O₃, MgO, Na₂O and MnO).

Table 3

Major elements content in phosphate sludge waste and their different phases issues of separating process (in %)
	SiO₂	CaO	MgO	K₂O	Na₂O	Al₂O₃	Fe₂O₃	MnO	P₂O₅	F	LOI
Phosphate sludge waste (N = 9)
Min	26.44	19.34	0.96	0.30	1.15	13.96	6.37	0.03	8.37	0.70	14.89
Max	27.93	20.63	1.01	0.32	1.24	15.60	6.86	0.06	10.61	2.65	15.51
Mean	27.34	20.13	0.99	0.31	1.21	14.94	6.61	0.04	9.09	1.55	15.25
Apatite-rich phases (N = 9)
Min	15.01	25.56	0.80	0.30	1.22	12.42	5.35	0.01	16.20	1.16	10.21
Max	24.17	27.44	0.88	0.32	1.36	14.28	6.46	0.06	20.60	4.04	11.22
Mean	21.34	26.11	0.84	0.32	1.28	13.78	6.04	0.03	17.27	2.27	10.70
Clay-rich phases (N = 9)
Min	29.98	12.72	1.11	0.30	1.09	14.40	6.80	0.03	0.55	0.04	19.23
Max	38.95	15.00	1.16	0.32	1.19	16.92	8.01	0.07	2.03	1.26	20.36
Mean	33.33	14.12	1.13	0.31	1.13	16.10	7.18	0.05	0.90	0.82	19.79

The contents of trace elements in phosphate sludge and their phases are given in Table 4. Variations in trace element contents are observed in the phosphate sludge waste: Cr (198.35-244.72 mg/kg), Cd (10.99–15.53 mg/kg), Pb (13.28–20.02 mg/kg), Cu (46.85–58.29 mg/kg), Co (40.07–57.84 mg/kg), Zn (509.08-593.97 mg/kg), Ni (110.05-142.02 mg/kg) and Hg (0.05–0.80 mg/kg). In the apatite-rich phases, the fluctuating levels are Cr (155.27-249.45 mg/kg), Cd (12.43–20.21 mg/kg), Pb (8.36–18.41 mg/kg), Cu (42.62–56.45 mg/kg), Co (37.34–67.56 mg/kg), Zn (453.84-610.02 mg/kg), Ni (94.77-139.33 mg/kg) and Hg (0.06–0.78 mg/kg); and in the clay-rich phases, Cr (211.48–265.60 mg/kg), Cd (9.06–14.58 mg/kg), Pb (10.26–21.37 mg/kg), Cu (47.44–60.56 mg/kg), Co (41.51–69.36 mg/kg), Zn (425.60-668.06 mg/kg), Ni (109.84-147.99 mg/kg) and Hg (0.04–1.25 mg/kg). A comparison of average trace element contents in phosphate sludge waste and in the different phases is shown in Fig. 4.

The clay-rich phases recorded higher levels of Cr, Pb, Cu, Co, Zn, Ni and Hg than the apatite-rich phases. On the other hand, Cd levels were higher in the apatite-rich phases than in the clay-rich phases.

Table 4

Trace element content in phosphate sludge waste and their different phases issues of separating process (in mg/kg)
	Cr	Cd	Pb	Cu	Co	Zn	Ni	Hg
Phosphate sludge waste (N = 9)
Min	198.35	10.99	13.28	46.85	40.07	509.08	110.05	0.05
Max	244.72	15.53	20.02	58.29	57.84	593.97	142.02	0.80
Mean	223.37	13.99	17.06	52.78	51.99	563.86	126.55	0.35
Apatite-rich phases (N = 9)
Min	155.27	12.43	8.36	42.62	37.34	453.84	94.77	0.06
Max	249.45	20.21	18.41	56.45	67.56	610.02	139.33	0.78
Mean	212.51	16.04	14.96	49.85	48.80	541.06	121.84	0.21
Clay-rich phases (N = 9)
Min	211.48	9.06	10.26	47.44	41.51	425.60	109.84	0.04
Max	265.60	14.58	21.37	60.56	69.36	668.06	147.99	1.25
Mean	234.24	11.85	18.73	53.84	54.68	585.70	130.28	0.42

Phases dispersion and enrichment factor associates

Clay phases generally have a higher enrichment factor than phosphate-mud wastes and apatite-rich phases (Table 5).

Table 5

Enrichment factors for phosphate sludge waste and the various phases expressed from the values for Shales (F1), world phosphorites (F2) and Togo (F3)
	Cr	Cd	Pb	Cu	Co	Zn	Ni	Hg
Phosphate sludge waste (N = 9)
Median	223.79	14.52	16.93	51.98	53.24	561.24	125.31	0.26
Shales 1	90.00	0.30	20.00	45.00	19.00	95.00	68.00	0.0004
Phosphorite 2	125.00	18.00	50.00	75.00	7.00	195.00	53.00	n.a
Phosphorite 3	356.00	44.00	67.00	158.00	2.5*	465.00	109.00	n.a
F1	2.49	48.39	0.85	1.16	2.80	5.91	1.84	653.25
F2	1.79	0.81	0.34	0.69	7.61	2.88	2.36	n.a
F3	0.63	0.33	0.25	0.33	21.30	1.21	1.15	n.a
Apatite-rich phases (N = 9)
Median	220.58	15.58	15.76	51.06	48.36	556.54	128.71	0.14
Shales 1	90.00	0.30	20.00	45.00	19.00	95.00	68.00	0.0004
Phosphorite 2	125.00	18.00	50.00	75.00	7.00	195.00	53.00	n.a
Phosphorite 3	356.00	44.00	67.00	158.00	2.5*	465.00	109.00	n.a
F1	2.45	51.94	0.79	1.13	2.55	5.86	1.89	359.30
F2	1.76	0.87	0.32	0.68	6.91	2.85	2.43	n.a
F3	0.62	0.35	0.24	0.32	19.34	1.20	1.18	n.a
Clay-rich phases (N = 9)
Median	239.99	12.31	19.79	52.68	54.04	600.62	126.59	0.20
Shales 1	90.00	0.30	20.00	45.00	19.00	95.00	68.00	0.0004
Phosphorite 2	125.00	18.00	50.00	75.00	7.00	195.00	53.00	n.a
Phosphorite 3	356.00	44.00	67.00	158.00	2.5*	465.00	109.00	n.a
F1	2.67	41.04	0.99	1.17	2.84	6.32	1.86	502.81
F2	1.92	0.68	0.40	0.70	7.72	3.08	2.39	n.a
F3	0.67	0.28	0.30	0.33	21.62	1.29	1.16	n.a
n.a = not available; F = Enrichment factor = Metal median (ppm)/shale or phosphorite (ppm); F1: shales [24], F2: world phosphorites [1] and F3: Togo phosphorites [14; *16]

High enrichment factors are observed for a number of trace elements in all three phases: Cr (F1, F2), Cd (F1), Cu (F1), Co (F1, F2, F3), Zn (F1, F2, F3), Ni (F1, F2, F3) and Hg (F1). When the different phases are transported to sea, the less dense clay-rich phases are likely to be transported further and cause much more damage to the marine environment than the apatite-rich phases, depending on the conditions prevailing in the marine environment.

K_d values for phosphate sludge waste and different phases

Particles can act as binding sites for contaminants in soluble form. For this reason, we carried out a study of the apparent partition coefficient (K_d). The values of this K_d are listed in Table 6 and compared with the values recommended for marine sediments and those obtained for phosphates from Togo (Table 7). The K_d values for elements such as Al, Fe, Cr, Cd, Pb, Cu, Zn, Ni and Hg are 4.15×10⁴, 1.97×10⁴, 4.74×10², 2.61×10², 0.60×10², 10.85×10³, 3.80×10³, 5, 17×10² and 9.23 in muddy phosphate wastes and 5.72×10⁴, 1.75×10⁴, 4.40×10², 6.33×10², 0.56×10², 4.07×10³, 4.88×10³, 4.39×10² and 8.29 in apatite-rich phases. In the clay phases, on the other hand, they are 24.50×10⁴, 2.02×10⁴, 5×10², 5.18×10², 0.93×10², 9.55×10³, 4.68×10³, 4.93×10² and 13.02 respectively. Clay-rich phases show a higher apparent partition coefficient for all elements studied, with the exception of Cd, which is higher in the apatite-rich phase. Cd is an integral part of the apatite structure.

The K_d values obtained for all phases studied are lower than those recommended for marine sediments and those obtained for Togo phosphates for valid elements [13, 25].

Table 6

K_d values for phosphate sludge waste and the various phases
	Al	Fe	Cr	Cd	Pb	Cu	Zn	Ni	Hg
	K_d ×10⁴	K_d ×10⁴	K_d ×10²	K_d ×10²	K_d ×10²	K_d ×10³	K_d ×10³	K_d ×10²	K_d
				Phosphate sludge waste (N = 9)
Min	3.92	1.73	3.69	1.93	0.36	4.12	2.31	3.82	0.56
Max	4.35	2.53	6.11	3.16	1.27	18.79	4.52	6.24	37.14
Mean	4.15	1.97	4.74	2.61	0.60	10.85	3.80	5.17	9.23
				Apatite-rich phases (N = 9)
Min	3.75	1.48	2.21	4.11	0.24	2.57	3.46	3.23	0.60
Max	7.99	2.04	5.58	8.76	0.78	7.70	7.27	6.45	37.64
Mean	5.72	1.75	4.40	6.33	0.56	4.07	4.88	4.39	8.29
				Clay -rich phases (N = 9)
Min	15.32	1.75	3.27	4.10	0.41	2.65	3.08	3.88	0.47
Max	40.58	2.45	7.23	6.83	2.24	19.13	5.62	6.15	44.13
Mean	24.50	2.02	5.00	5.18	0.93	9.55	4.68	4.93	13.02

Table 7

Comparison of K_d values obtained with recommended K_d values for coastal sediments and K_d values for Hahotoé-Kpogamé phosphorite
	Al	Fe	Cr	Cd	Pb	Cu	Zn	Ni	Hg
	K_d ×10⁴	K_d ×10⁴	K_d ×10²	K_d ×10²	K_d ×10²	K_d ×10³	K_d ×10³	K_d ×10²	K_d
				Phosphate sludge waste (N = 9)
Min	3.92	1.73	3.69	1.93	0.36	4.12	2.31	3.82	0.56
Max	4.35	2.53	6.11	3.16	1.27	18.79	4.52	6.24	37.14
Mean	4.15	1.97	4.74	2.61	0.60	10.85	3.80	5.17	9.23
				Apatite-rich phases (N = 9)
Min	3.75	1.48	2.21	4.11	0.24	2.57	3.46	3.23	0.60
Max	7.99	2.04	5.58	8.76	0.78	7.70	7.27	6.45	37.64
Mean	5.72	1.75	4.40	6.33	0.56	4.07	4.88	4.39	8.29
				Clay -rich phases (N = 9)
Min	15.32	1.75	3.27	4.10	0.41	2.65	3.08	3.88	0.47
Max	40.58	2.45	7.23	6.83	2.24	19.13	5.62	6.15	44.13
Mean	24.50	2.02	5.00	5.18	0.93	9.55	4.68	4.93	13.02
			K_d for phosphorite of Hahotoé-Kpogamé^a
Mean	4.7× 10⁵	2.5× 10⁵	n.a	1.2× 10³	n.a	n.a	5.7× 10³	8× 10³	n.a
			Recommended K_d for costal sediment^b
Mean	n.a	3× 10⁸	5× 10⁴	3× 10⁴	1× 10⁵	n.a	7× 10⁴	2× 10⁴	4× 10³
n.a = not available; ^a[13] ; ^b[25]

Parameters such as redox conditions and pH control the chemistry of these phosphate-mud mine wastes and their various phases. The clay-rich phases are in a higher acidity and oxidizing condition than the apatite-rich phases, which would be more conducive to the mobilization of these various pollutants.

Phosphates have a great capacity to store large quantities of trace elements such as Cr, Zn, Cd, Cu, Ni, Pb, Co [1, 14–15]. These high concentrations are reflected in phosphate sludge discharged into the sea [5, 19]. Some of these trace elements are preferentially associated with the detrital phase, others with continental inputs, and some with organic matter, while others are an integral part of the apatite structure. Anthropogenic pollutants such as metals, organic matter and nutrients are associated with particulate and dissolved element inputs to natural waters. The clay-rich fraction absorbs more heavy metals than the apatite-rich fraction due to its high surface area and surface chemistry; except for Cd, which is an integral part of the apatite structure. This explains its higher content in apatite-rich phases. The distribution of elements in the different phases during this experiment highlights the phases of trace element preference and association during transport once the waste has been discharged into the sea. It also provides a link between the concentrations of trace elements in the different phases of phosphates mined in south-east Togo and coastal marine sediments highlighted by Gnandi's work [14].

Other secondary minerals, such as aluminum and iron oxides, are more prevalent in the clay-rich phase than in the apatite-rich phase. Most anthropogenic contaminants are associated with this clay phase. Depending on marine environmental conditions such as wind, tidal currents, etc., the transport of dense apatite-rich phases will be a few meters; for less dense clay-rich phases, on the other hand, it will be a few kilometers.

The higher acidity and oxidizing conditions of the clay phases would be more conducive to the mobilization of these various pollutants once the sludgy phosphate waste is discharged into the sea. Muddy phosphate waste is highly enriched in trace elements. Cd accumulates preferentially in dense, rich phases. But other trace elements such as Cr, Pb, Cu, Co, Zn, Ni and Hg accumulate more in clay-rich phases. The conditions under which the lighter, clay-rich phases occur make them much more toxic once each phase is resuspended and transported by tidal currents to the sea. These lighter phases will be transported for miles.

The authors declare that they received no funding to conduct this study. They have no conflicting interest relevant to the content of this article.

Author Contribution

K.G. supervised the study. D.S. and K.G. designed the study and collected samples in the field; they carried out the experimentation, physicochemical analyses, data processing and drafting of the manuscript. P.K. and M.G. contributed to collecting samples in the field and carrying out the experimentation, physicochemical analyses, data processing and drafting of the manuscript. L.L. and A. G. G. contributed to the physicochemical analyses, data processing and drafting of the manuscript.

Acknowledgements

Our sincere thanks to AWAS GmbH for providing us with the mini hydrocyclone Galaxi-Tower, which enabled us to carry out the separation of the different phases.

Data availability

All data generated in the course of this study may be distributed but on request to the corresponding author.

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No competing interests reported.

Geochemical characterization of different phases of phosphate sludge waste according to their density in seawater

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

Results

Physicochemical properties of phosphate sludge waste and its various phases

Geochemical composition

Phases dispersion and enrichment factor associates

K_d values for phosphate sludge waste and different phases

Discussions

Conclusions

Declarations

Author Contribution

Acknowledgements

Data availability

References

Additional Declarations

Status:

Version 1

Geochemical characterization of different phases of phosphate sludge waste according to their density in seawater

Status:

Version 1

Abstract

Figures

Introduction

Materials and methods

Results

Physicochemical properties of phosphate sludge waste and its various phases

Geochemical composition

Phases dispersion and enrichment factor associates

Kd values for phosphate sludge waste and different phases

Discussions

Conclusions

Declarations

Author Contribution

Acknowledgements

Data availability

References

Additional Declarations

Status:

Version 1

K_d values for phosphate sludge waste and different phases