Yellow phosphorus is used widely in the world for production of phosphoric acid, various phosphates, flame retardant, detergent, water treatment, metal surface treatment, etc. After the production of yellow phosphorus, a large amount of phosphorus sludge is discharged to environment, causing environment pollution. This work focused on treatment of yellow phosphorus slag (YPS) and application of it as an absorbent for chromium (VI) ion and methylene blue. The YPS was first washed with water to remove phosphoferrite (FeP) and impurities and then being milled and transferred to a float sorting system to obtain YPS particles. The characteristics of YPS particles were determined by inductively coupled plasma-mass-spectrometry (ICP-MS), energy-dispersive X-ray spectroscopy (EDX), infrared spectroscopy (IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), zeta potential, and nitrogen adsorption methods. The YPS particles were retreated with NaOH, HNO3, and EDTA solutions to evaluate the influence of these agents on metal ion and organic compound adsorption ability by YPS. The adsorption parameters of chromium (VI) ion and methylene blue (MB) by treated YPS particles were recognized by the ultraviolet-visible (UV-Vis) spectrometer. The effects of temperature, pH solution, and initial concentration of absorbed substances on the adsorption ability were investigated. The adsorption isotherms and adsorption kinetics of chromium (VI) ion and MB by YPS particles were also determined. The obtained results confirmed that the green technology used to treat the YPS2 particles is suitable to obtain an effective absorbent. The adsorption efficiency of YPS2 particles for removal of chromium (VI) ions is smaller than that for removal of MB in aqueous solutions. The adsorption isotherm of MB adsorption process is complied with the Langmuir isotherm while the adsorption kinetic fits well with the pseudo-second-order reaction model. The thermodynamic parameters of MB adsorption processed on YPS2 were calculated and discussed.
Yellow phosphorus slag (YPS) is a waste product of manufacturing yellow phosphorus (P4) using the electrical furnace method at 1400°C–1600°C . The worldwide yellow phosphorus production yields an average of 1.5 million tons of P4 each year . In which, the countries discharge a large YPS weight including Chinese (75%), Kazakhstan (13%), America (8%), Western Europe (6%), Russia (4%), and the rest of India. For each ton of manufactured P4, about 8 to 10 tons of yellow phosphorus slag are produced . This waste product is often left accumulated in the landfills, proposing a potential pollution thread to the environment. The majority of YPS particles compose of CaO and SiO2, accounting for approximately 85% of the slag mass . Other components of the YPS are Al2O3, Fe2O3, MgO, and some rare earth metals [4, 5]. In Vietnam, the P4 is mainly produced and supplied by Lao Cai Yellow Phosphorus JSC (Tang Loong Industrial Zone, Bao Thang District, Lao Cai Province, Vietnam), with the productivity of 93,800 tons P4/year and about 750,000–1,031,000 tons of YPS waste are produced and released directly into the surrounding environment . Therefore, it is important to find different ways to utilize this waste product. Recent studies have been focusing on applying YPS into asphalt or asphalt binder [7, 8], cemented backfilling , retarding the early hydration of cement  or recovering rare earth metals .
YPS had been treated by different technologies and methods [11–13]. In German Auslegeschrift No. 1,804,172, the YPS particles were heat treated at 1000°C to 1100°C in a rotary furnace . In German Offenlegungsschrift No. 2,211,505, a physical separation method, for example, by gravity separation with tetrabromoethane as the liquid phase, the refinement of silicate slag phase was reported . US patent No. 4,243,425 presented a method for refining of silicatic slag phase with steam at a temperature between 600°C and softening temperature of the slag .
In this work, an effective method for treatment of YPS was developed. This is a combination of the water washing flotation system with milling and sorting flotation. The waste water system was treated to recover P2O5, F, and minerals for other applications. The advantages of this technology were low energy consumption, reduction of noise and dust pollution, closed technology, and environmental friendly and waste water can be easily processed and reused (about 80–90%). The minimized weight of chemicals was used, and treatment process was carried in room temperature. This approached to the green technology, the general trend of the world.
The study on metal ion adsorption ability of YPS in aqueous solutions has been limited in the research. Herein, the YPS was chosen as an absorbent due to its silicate structure with high content of silica. Moreover, the small size particle as compared with micrometer inorganic additives is an advantage of the YPS. Hence, the obtained YPS could be expected to use as an absorbent for removing toxicity substances in aqueous solutions. In this paper, chromium (VI) and methylene blue have been chosen as the model substances to study the adsorption ability of YPS. Chromium, commonly found in various sources of industrial waste water, is one of the most toxic metal ions, which influences seriously the biodiversity of the environment and causes several health issues to humans. Chromium often comes from discharge of industrial activities such as leather tanning, electroplating, or textiles . It exists in water with various oxidation forms, but mainly in trivalent and hexavalent state , in which the hexavalent state is considered the most toxic because it is easily dilution and can bioaccumulate in human organs . Some studies reveal that Cr (VI) ions can cause digestive system and lungs cancer, gastrointestinal and dermatological problems, severe diarrhea, and hemorrhage [17–19]. Many materials have been utilized to adsorb Cr (VI) ions in aqueous solutions including banana peel , coffee resins , fly ash , groundnut hull , or zeolite NaX  and showed good results.
Dye has been an important ingredient in many common daily industries such as textile, leather, cosmetics, plastics, and food production . Nevertheless, colored dye waste water constitutes considerable issues to the environment and water sources . Complex aromatic molecular structures of dyes make them nondegradable . Dyes, which have high demand for chemical and biological oxygen, high toxicity, and capability to hinder sunlight penetration into water bodies, are harmful pollutants to the aquatic ecosystem . Methylene blue (MB), a cationic dye, is widely used in biology, medical science, chemistry, and dye field. However, long-term exposure to MB can cause certain symptoms and illnesses such as increased heart rate, vomiting, nausea, anemia, cyanosis, and tissue necrosis [29, 30]. Various adsorbents have been used to adsorb MB in aqueous solutions with promising output such as activated carbon , natural clay , sea grass dead leaves , or modified pumice stone .
According to our calculation, the price of one tone of YPS at the Tang Loong Industrial Zone is only about 22 USD. This indicates that YPS product is cost-effective and promising for application not only as an absorbent but also as an additive for other fields (composites, paint, coating, etc.). Acids and bases are used for treatment of inorganic waste, for example, fly ash and gypsum, to improve the adsorption ability of these waste products [18, 20, 22, 23]. Besides, EDTA is known as a chemical used for both industrial and medical purposes. In the industry, it is mainly used to sequester metal ions in aqueous solution. Thanks to its high affinity for metal ions, EDTA can enormously enhance the chelation properties of the adsorbent. Therefore, in this work, NaOH, HNO3, and EDTA were chosen for retreatment of YPS particles to evaluate their adsorption ability. We mainly focused on the characterization of YPS products as well as investigation of the factors affecting on the adsorption ability of YPS products for removal of chromium (VI) ions and methylene blue in aqueous solutions.
Yellow phosphorus slag (YPS0) is waste product of Lao Cai Yellow Phosphorus JSC (Tang Loong Industrial Zone, Bao Thang District, Lao Cai Province, Vietnam) with a particle size of 50–100 µm; density of 2.75 g/cm³; hardness of 1–3; compressive strength of 80–100 MPa; water absorption of 1–4%; and porosity of 10–12%. Ethylenediaminetetraacetic acid (EDTA); 1,5-diphenylcarbazide (DCP); potassium chromate; and methylene blue (MB) were purchased from Merck Co. HNO3 and NaOH are the commercial products which are used as received.
2.2. Surface Treatment of Yellow Phosphorus Slag
2.2.1. Treatment of Yellow Phosphorus Slag by Flotation System
Firstly, the washing flotation system was used to remove preliminary soluble impurities and to separate the particles for next stage. In this stage, the large content of FeP, P2O5, fluorine compounds, and some other impurities were removed and the YPS0 was then wet sieved down to prepare a granular material which is smaller than 100 µm in diameter (YPS1) (by sieve analysis). These particles could be applied for cement production or concrete directly. In the next stage, the particles were milled and transferred to a float sorting system to obtain particles in smaller size. The solid part was then rotated in a spinning double-drum composter and dried at 100°C until unchanged weight. The product was designed as YPS2 with the calcium silicate content in particles higher than 90%. The waste water system was treated to recover P2O5, fluorine compounds, and minerals for other applications. The BET surface area/t-plot micropore area, pore diameter, and pore volume of YPS2 product are 1.3145 m²g⁻¹/0.3024 m²g⁻¹, 21.4897 nm, and 0.000134 cm³g⁻¹, respectively (determined by the nitrogen adsorption method on a TriStar 3000 V6.07 A device).
2.2.2. Surface Treatment of Yellow Phosphorus Slag (YPS) Using NaOH and HNO3 Solutions
The YPS particles were retreated by NaOH or HNO3 solution as follows: 200 mL of NaOH 1 M (or HNO3 1 M) solution was added into a flask containing 20 g of YPS particles and stirred at 70°C for 3 hours. Then, the treated YPS particles by NaOH (or HNO3) solution were filtered and washed with distilled water until filtered aqueous solution reached to neutral medium (pH 7). After that, the treated YPS particles were dried in an oven at 100°C for 12 hours. The YPS0 particles and YPS2 particles which were treated with NaOH or HNO3 solution were designed as YPS0-NaOH, YPS2-NaOH, YPS0-HNO3, and YPS2-HNO3.
2.2.3. Surface Treatment of Yellow Phosphorus Slag (YPS) Using EDTA
In this research, YPS was modified by using EDTA as follows: 5 g of YPS was added into 100 mL distilled water in a 250 mL glass beaker containing 0.5 g of EDTA. This solution was then stirred on a magnetic machine at a speed of 500 rpm for 2 hours at 60°C. Next, the solution was filtered to obtain the solid part. Finally, the solid part was dried in an oven at 100°C for 3 hours.
2.3.1. Infrared Spectroscopy (IR)
IR spectra of YPS samples were recorded using a Nicolet iS10 spectrometer (Thermo Scientific, USA) in the range of wavenumbers from 4000 cm⁻¹ to 400 cm⁻¹, resolution of 8 cm⁻¹, and scan average of 32 times.
2.3.2. Energy-Dispersive X-Ray Spectroscopy (EDX)
EDX spectra of the YPS samples were carried out on a SEM/EDS device (Oxford Instruments, UK).
2.3.3. Inductively Coupled Plasma-Mass-Spectrometry (ICP-MS)
The element content of YPS samples was detected by NexION 2000 ICP-DRC-QMS (Perkin Elmer, USA).
2.3.4. Scanning Electron Microscopy (SEM)
Scanning electron microscopy (SEM) of YPS samples was taken on a SEM-S-4800 device (Hitachi, Japan). The samples were coated a Pt layer on the surface to enhance the resolution of images.
2.3.5. X-Ray Diffraction Analysis (XRD)
XRD patterns of YPS samples were performed on a Siemens D5000 X-ray diffractometer (XRD) with CuKα radiation source (λ = 0.154 nm) at 40 kV generator voltage with 0.03° step and 30 mA current by 0.043°/s scan speed in the range of 2θ from 2° to 70°.
2.3.6. Zeta Potential
Zeta potential of the YPS2 sample was conducted on a Zetasizer, ver. 6.2, Malvern Instruments, with zeta runs of 12, count rate (kcps) of 288.7, and measurement position of 4.5 mm at 25°C. The YPS2 was dispersed in water (pH ≈ 7) with its dispersant RI of 1.330, viscosity of 0.8872 cP, and dispersant dielectric constant of 78.5.
2.3.7. Ultraviolet-Visible (UV-Vis) Absorption Spectrometry
Spectra of samples were determined on a UV spectrophotometer (CINTRA 40, GBC, USA) in the range of wavelength from 200 to 800 nm.
2.4. Determination of Adsorption Ability of Yellow Phosphorus Slag in Aqueous Solution
An exact weight of the YPS samples was added into a 100 mL of Cr (VI) or MB solution. The solution was stirred on a magnetic stirrer at room temperature for 120 minutes. The solution was then filtered, and 25 mL of aliquots was withdrawn. For MB adsorption, the withdrawn solution was monitored by a UV-Vis spectrophotometer (CINTRA 40, GBC, USA) at λmax = 664 nm. For Cr (VI) adsorption, 1 mL of H2SO4 1M and 1 mL of DCP 0.5% solution were introduced into the withdrawn solution and this solution was kept for 10 minutes before taking on a UV-Vis spectrophotometer at λmax = 540 nm. All studies were performed in triplicate to increase accuracy.
2.5. Determination of Cr (VI) Ion and Methylene Blue Adsorption Isotherms and Adsorption Kinetics of Yellow Phosphorus Slag
2.5.1. Adsorption of Cr (VI) Ions and Methylene Blue Using YPS Samples
The amount of adsorbate per amount of adsorbent at equilibrium condition, Q (mg/g), was calculated as follows:where and are the concentration of adsorbate in solution at initial and equilibrium (mg/L), V is the solution volume (L), and W is the mass of YPS samples (g).
The percentage of metal ions removed, H (%), was calculated using the following equation:
2.5.2. Adsorption Isotherms
In this work, we study the adsorption behavior in the solid-liquid system using four adsorption isotherms: Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich isotherms. Langmuir isotherm equation for ion adsorption: where is the maximum monolayer adsorption capacity (mg/g) and kL is the Langmuir isotherm constant representing binding energy of the adsorption system (L/mg). Freundlich isotherm equation: where is the Freundlich isotherm constant (mg/g) indicating adsorption capacity and is adsorption intensity. Temkin isotherm equation: where is the Temkin isotherm equilibrium binding constant (L/g), is the Temkin isotherm constant related to heat sorption (J/mg), T is absolute temperature (K), and R is the gas constant (8.314 J/mol/K). Dubinin–Radushkevich (DR) isotherm equation: where is the theoretical isotherm saturation capacity (mg/g) and is the Dubinin–Radushkevich isotherm constant (mol²/kJ²).
2.5.3. Adsorption Kinetics
Adsorption kinetics were studied using four reaction models: first-order, pseudo-first-order, second-order, and pseudo-second-order reaction models. First-order reaction model: Pseudo-first-order reaction model: Second-order reaction model: Pseudo-second-order reaction model: where is the maximum monolayer coverage capacity (mg/g); and are the amount of adsorbate adsorbed per gram of adsorbent at equilibrium time and testing time t (mg/g); and are the solution concentration at the initial time and the testing time t (mg/l); and are the rate constant (per minute) of the first-order reaction model, pseudo-first-order reaction model, second-order reaction model, and pseudo-second-order reaction model, respectively.
3. Results and Discussion
3.1. Characteristics of Yellow Phosphorus Slag (YPS) Samples
3.1.1. FTIR Spectra of YPS Samples
Figure 1 shows FTIR spectra of the YPS samples. It can be seen that the absorption peaks of YPS samples mostly appeared in the wavenumber range of 1500–500 cm⁻¹. In the low frequency region, the deformation vibration of bridge Si–O–Si and terminal O–Si–O groups and the metal-oxygen polyhedral (CaOn) results in the absorption bands of about 550 cm⁻¹. The adsorption peak at 699 cm⁻¹ assigned to the Al–O and Si–O bonds and showed the presence of pseudowollastonite α–CaSiO3 in the slag. The spectra from 750 cm⁻¹ to 1050 cm⁻¹ with the peak at 872 and 920 cm⁻¹ correspond to the stretching vibration of Si–O groups, indicating the presence of the glass-like earth silicon. Numerous unbridged connections of Si–O emerging are the result of breach of the polymer network Si–O–Si of the frame due to the introduction of Са atoms for replacement of Si atoms in the Si–O–Si network. Furthermore, the peaks at 1414 and 1484 cm⁻¹ represent the vibration of in calcite, one of main compositions of YPS samples. This result is similar to the report of Zinesh et al. .