Enhancing Arsenic Solidiﬁcation/Stabilisation Efﬁciency of Metallurgical Slag-Based Green Mining Fill and Its Structure Analysis

: To dispose of arsenic-containing tailings with low carbon and high efﬁciency, sodium sulphate (Na 2 SO 4 ), sodium hydroxide (NaOH), calcium nitrate Ca(NO 3 ) 2 and calcium hydroxide Ca(OH) 2 were independently added to metallurgical slag-based binder (MSB) solidification/stabilisation (S/S)-treated tailings (MSTs) to enhance the MST arsenic S/S performance. Results showed that only Ca(OH) 2 could increase the unconﬁned compressive strength of MST from 16.3 to 20.49 MPa and decrease the leachate As concentration from 31 µ g/L to below 10 µ g/L. Na 3 AsO 4 · 12H 2 O and NaAsO 2 were used to prepare pure MSB paste for mechanism analysis. The results of microstructure analyses showed the high speciﬁc surface area and amorphous properties of calcium–sodium aluminosilicate hydrate facilitated the adsorption or solid-solution formation of As(V) and As(III). As(V) formed an inner-sphere complex in ettringite, whereas As(III) formed an outer-sphere complex, and the relatively larger size and charge of As(V) compared with SO 42 − restrict substitution inside channels without affecting the ettringite structure under high loading of As(V). The added Ca(OH) 2 promoted the hydration reaction of MSBs and facilitated the formation of a Ca–As(V) precipitate with low solubility, from Ca 4 (OH) 2 (AsO 4 ) 2 · 4H 2 O ( K sp = 10 − 27.49 ) to Ca 5 (AsO 4 ) 3 (OH) ( K sp = 10 − 40.12 ). This work is beneﬁcial for the application of cement-free MSB in the S/S process.


Introduction
Arsenic (As) is a toxic and carcinogenic element, which can cause severe effects on human health, such as skin, lungs, kidney and liver cancers, even at a low concentration [1]. The mining, processing and smelting industry of nonferrous metals is the most widespread source of As emissions in China, accounting for 61.8% [2]. Mining wastewater, smelting slag, dust and tailings are piled up in tailings ponds, which have potential environmental risks and occupy a large percentage of soil [3]. Heavy metal pollution from a large number of tailings poses a major threat to the environment [4,5]. The treatment of heavy metal pollution has received increasing attention since the implementation of the Soil Pollution Prevention and Control Law of the People's Republic of China in 2019 [6]. It is imperative to develop effective and economically viable technologies to reduce pollution from the nonferrous metal industry.
Solidification/stabilisation (S/S) technology has been used to dispose of metal(loid)containing waste by forming a less soluble, mobile or toxic product through adsorption, co-precipitation and physical encapsulation [7]. Ordinary Portland cement (OPC) is a Metals 2021, 11,1389 2 of 13 traditionally used binder in S/S processes, while the high carbon footprint of cement clinker manufacture is of concern [8,9]. To achieve an environment-friendly binder for the S/S of As-containing waste, many studies have been conducted to replace cement in part or whole using materials, such as red mud, blast furnace or metakaolin-assisted cement [1,10], cement-free clay-based binders [11], red mud-enhanced magnesium phosphate cement [12], pine sawdust biochar [13], coal fly ash [14], zero-valent iron or magnetic biochar [15]. However, there have been few reports on the use of cement-free metallurgical slag-based binders (MSB) [16,17], which consist of steel slag powder (SSP), ground-granulated blast furnace slag (GGBFS) and flue gas desulphurisation gypsum (FGDG). Steel slag is a byproduct of a steel-making process, known as overburnt clinker because its mineral phase composition is similar to that of cement clinker [18]. In 2019, 149.45 million tonnes of it were produced in China; however, the usage rate is less than 40% [19]. A large accumulation of steel slag pressurises the environment [20]. GGBFS is a by-product of the steel-making industry and is used as a supplementary cementitious material. In the hydration process of MSB, the relatively high alkalinity of SSP facilitates the breakage of Si-O and Al-O bonds in the GGBFS vitreous structure to form SiO 4 4− and AlO 4 5− [21,22], which react with dissolved Ca 2+ to form calcium silicoaluminate hydrate (C-A-S-H) gels [11,23]. The AlO 4 5− dissolved in solution transforms from four coordinated aluminium ions to six coordinated aluminium ions [Al(OH) 6 ] 3− in alkaline environments, which reacted with the Ca 2+ and SO 4 2− dissolved from gypsum to form ettringite [24]. In this study, As-containing tailings were chosen as the S/S objects, as other hazardous elements' leaching risk is very low after S/S treatment (Table S1). In order to dispose of As-containing tailings with low carbon and high efficiency, the MSB (60 wt% GGBFS, 30 wt% SSP, and 10 wt% FGDG) was chosen as the binder for the As-containing tailings, based on a previous study [25]. SO 4 2− (Na 2 SO 4 ), OH − (NaOH), Ca 2+ (Ca(NO 3 ) 2 or Ca 2+ and OH − (Ca(OH) 2 ) was added to the MSB-S/S treated tailings (MST) to enhance the MST-As-S/S performance. Arsenic in tailing occurs in various states in the natural environment, and its main forms in regard to leaching and migration characteristics are soluble arsenite As(III) and arsenate As(V) compounds [26]. The As-S/S mechanism of MSB and the mechanism of efficiency improvement are both unclear; therefore, Na 3 AsO 4 ·12H 2 O and NaAsO 2 were used to study the S/S mechanisms through X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscope-energy dispersive spectrometer (SEM-EDS) analysis. This study will improve the engineering application prospects of MSB and reusability of MST.

As-Containing Tailings and Metallurgical Slag-Based Binders
As-containing tailings were collected from a closed lead-zinc tailings pond in Hechi Nandan, Guangxi (China). The main components of the tailings were quartz, calcite and fluorite, of which the D 50 and D 90 particle sizes were 42 and 188 µm, respectively ( Figure S1). The total concentration of As in the tailings was 2098 mg/kg, as determined using an Agilent 7500a ICP-MS (Agilent Technologies, Santa Clara, CA, USA) after aqua regia digestion (1/3 concentrated HNO 3 and HCl (v/v)) based on Chinese standard method HJ 803-2016. The leaching concentration of As in the tailings after horizontal oscillationleaching tests was 0.57 mg/L, based on the Chinese standard method HJ 557-2010 (Table 1). The MSB consisted of GGBFS, SSP and FGDG. The chemical components of raw materials are shown in Table 1, and the XRD spectra of the binders are shown in Figure S2. The analytical reagents, Na 2 SO 4 , NaOH, Ca(NO 3 ) 2 and Ca(OH) 2 , were used as added additive for the binders, respectively. A polycarboxylic acid water-reducer (WR) was used to achieve an acceptable flowability of mortar during S/S treatment. The analytical reagents, Na 3 AsO 4 ·12H 2 O and NaAsO 2 , were used to prepare the pure MSB paste to study the S/S mechanisms of As(III) and As(V).

Cement-Free S/S Treatment
For the preparation of MST mortar samples, the different constituents of binders were mixed with tailings at specific ratios (Table 2). MSB with 60 wt% GGBFS, 30 wt% SSP and 10 wt% FGDG was chosen due to its high compressive strength, according to the findings of previous studies [25]. Four additives (Na 2 SO 4 , NaOH, Ca(NO 3 ) 2 and Ca(OH) 2 ) were incorporated into the MSB mass individually at 5 wt%, 10 wt% and 15 wt%. The binders-to-tailings (B/T), water-to-solids (W/S) and WR-to-solids (WR/S) mass ratios were maintained at 0.25, 0.2 and 0.008, respectively, in all MST samples. After the tailings and binders were mixed for 1 min using a standard cement mortar mixer, distilled water used to dissolve the WR was added, and the blended mortar was mixed for another 5 min, before being cast into steel moulds (40 mm × 40 mm × 160 mm) to prepare the MST samples. To prepare MSB paste samples, 5 wt% concentrations of the analytical reagent, Na 3 AsO 4 ·12H 2 O or NaAsO 2 , was added (Table 2), which were then mixed with water until the mass ratio of water-to-binders (W/B) reached 0.35. The paste samples were stirred for 5 min and then cast into steel moulds (30 mm × 30 mm × 50 mm). All mortar and paste samples were cured in a moist cabinet at (40 ± 2) • C and (90 ± 1)% relative humidity (similar to the underground filling environment in Guangxi [27]) for 3 day, and then the samples were demoulded and placed under the same curing conditions until the appropriate degree of sample ageing was achieved.

S/S Performance and Spectroscopic/Microscopic Analysis
The S/S performance was evaluated in terms of the unconfined compressive strength (UCS) and As leachability of the MST samples. The UCS of the samples was assessed on the basis of the Chinese standard GB/T 17671-1999 method of testing cements. The determination of UCS on three replicates at each curing time, and the average value with standard deviations of less than 5% was reported. The leachability of MST was assessed using the Chinese standard method HJ 557-2010, wherein the ratio of leachant (deionised water) volume to solid specimen weight (crushed to size < 3mm) was maintained at 10:1 (L/kg). Leachate samples were oscillated horizontally at 110 ± 10 times/min for 8 h and then left to stand at room temperature for 16 h. The pH values were measured, and the leachate was filtered through a 0.45-µm polypropylene microfiltration membrane. Afterwards, the filtered leachate samples were acidified to below pH 2 using concentrated HNO 3 and then stored at 4 • C in the dark before As detection using a PerkinElmer Optima 8300 ICP-OES (Perkin Elmer, Cumberland, NJ, USA) or an Agilent 7500a ICP-MS (Agilent Technologies, Santa Clara, CA, USA), with all samples analysed in triplicate to ensure accuracy. Mean results for triplicate sample analysis (with standard deviations of less than 5%) and error bars, are presented in this study. The As curing rate was evaluated using Equation (1): where C s/s is the As leaching concentration of MST, and µg/L; C T is the As leaching concentration of untreated tailings.  The hydration of pulverised and sieved paste samples was terminated using alcohol drenching at the specific testing age, with samples then dried at 50 • C under a vacuum for 24 h for further characterisation. Mineralogy was detected using a high-resolution powered Rigaku D/max-RB XRD with CuKα radiation, a voltage of 40 kV, current of 200 mA and 2θ scanning, ranging between 10 • and 70 • . The morphology and elemental mapping of the samples were analysed using SEM-EDS (JSM-6701F). Structural and chemical bond analyses were performed using GX Perkin-Elmer FTIR (Perkin Elmer, Cumberland, NJ, USA) between wavelengths of 400-4000 cm −1 and at a resolution of 0.125 cm −1 .

The S/S Performance of MST Samples
The UCS results for MST samples (Figure 1a) demonstrate that MST combined with various additives performed differently. The 3, 7 and 28 day UCS results for T were 11, 13.8 and 16.3 MPa, respectively. An increase in concentrations of Na 2 SO 4 , NaOH and Ca(NO 3 ) 2 reduced the USC of MST at every curing time, especially for the addition of NaOH, which inhibits hydration reactions and reduced the USC of MST (the lowest 28 day USC (T-H3 3. ples after the horizontal oscillation-leaching experiments. The leachate As concentration of T samples was 31 μg/L, which exceeded the prescribed As concentration limit value in the Chinese standard GB 5749-2006 for drinking water quality (10 μg/L, the black line in Figure 1b), indicating a relevant risk of environmental pollution. The addition of Na2SO4, NaOH or Ca(NO3)2 enhanced leachate As concentration, particularly that of T-H3, which reached 530 μg/L. The addition of Ca(OH)2 reduced the leachate As concentration, particularly that of T-C2 and T-C3, which was less than 10 μg/L.

XRD Pattern Analysis
The XRD patterns of MSB paste hydrated for 3 and 28 day are presented in Figure  2a Figure 1b), indicating a relevant risk of environmental pollution. The addition of Na 2 SO 4 , NaOH or Ca(NO 3 ) 2 enhanced leachate As concentration, particularly that of T-H3, which reached 530 µg/L. The addition of Ca(OH) 2 reduced the leachate As concentration, particularly that of T-C2 and T-C3, which was less than 10 µg/L.

XRD Pattern Analysis
The XRD patterns of MSB paste hydrated for 3 and 28 day are presented in Figure 2a,b), respectively. The main mineral phases of the B sample, as shown in Figure 2a Figure S2). With the addition of As(III) (sample B(III)), the peaks of ettringite decreased. However, when As(V) was added (B(V)), the peaks of ettringite disappeared due to the formation of a new phase, (Ca 4 Al 2 SO 10 ·16H 2 O), and the peaks of Ca-As coprecipitation Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O appeared.
Compared with Figure 1a, the added Ca(OH)2 characteristic peaks appeared in the XRD patterns of B-C3 and B-C3-(III); however, it did not appear in B-C3(V) (Figure 2b), and the peaks of Ca-As co-precipitation Ca5(AsO4)3OH·xH2O appeared.  Compared with Figure 1a, the added Ca(OH) 2 characteristic peaks appeared in the XRD patterns of B-C3 and B-C3-(III); however, it did not appear in B-C3(V) (Figure 2b), and the peaks of Ca-As co-precipitation Ca 5 (AsO 4 ) 3 OH·xH 2 O appeared.   (Figure 3a 3 -6 ), which was due to the As-O stretching vibration of the As(III) and As(V) species [28,29]. Figure 3b display the FTIR spectra of the B-C3, B-C3(III) and B-C3(V) samples, respectively. These spectra are all similar to those presented in Figure 3a, except that the spectra of the B-C3 and B-C3(III) samples exhibited sharp adsorption bands at 3642 cm −1 due to the addition of Ca(OH) 2 . However, the sharp adsorption bands at 3642 cm −1 did not appear in B-C3(V), illustrating that Ca(OH) 2 reacted with As(V). Bands around 1110 cm −1 , corresponding to the S-O 4 stretching mode (v3) in Figure 3b 1 -6 , were offset at 1114 cm −1 in B-C3 sample, 1111 cm −1 in B-C3(III) and 1117 cm −1 in B-C3(V), indicating that, after As(III) and As(V) were added, the hydration products associated with SO 4 2− were affected, as shown by the XRD pattern (Figure 2b).  Table S2 (b-1, b-2 and b-3). After As(V) was added, the needle-like ettringite structure was undetected (Figure 4c,d), and a small amount of As(V) was found in the short columnar crystals of sodium sulphate, combined with the component analysis results (Table S2 (d-1)). Moreover, according to the SEM-EDS mapping images ( Figure S3) of the red box in Figure 4d, the distribution of As(V) was almost similar to that of Ca, Si and Al, and the distribution of Na was similar to that of S. Therefore, the majority of the added As(V) entered the C-A-S-H gels through isomorphism substitution or was adsorbed and encapsulated by the gels, whereas the tiny minority of that entered the sodium sulphate to form sodium arsenic alunite. After As(III) was added, combined with the component analysis results (Table S2 (e-1,f-3)), different amounts of As(III) were detected in the C-A-S-H, ettringite, Na 2 SO 4 and Na 2 Ca(SO 4 ) 2 (Figure 4e,f).  Table S3, which may be due to their content being lower than the detection limit of the XRD technique; both kinds of flakes disappeared as the hydration reaction proceeded.

SEM-EDS Analysis
Al, and the distribution of Na was similar to that of S. Therefore, the majority of the added As(V) entered the C-A-S-H gels through isomorphism substitution or was adsorbed and encapsulated by the gels, whereas the tiny minority of that entered the sodium sulphate to form sodium arsenic alunite. After As(III) was added, combined with the component analysis results (Table S2 ( e-1,f-3)), different amounts of As(III) were detected in the C-A-S-H, ettringite, Na2SO4 and Na2Ca(SO4)2 (Figure 4e,f).   Table S3, which may be due to their content being lower than the detection limit of the XRD technique; both kinds of flakes disappeared as the hydration reaction proceeded.

Discussion
This study demonstrates the different As S/S performance of MST samples after four additives (Na2SO4, NaOH, Ca(NO3)2 and Ca(OH)2) were added, as shown in Table  S4. The MSB is selective for additives and only Ca(OH)2 exerted a positive effect on the As S/S performance. The addition of Na2SO4 had little effect on the pH values, UCS and As curing rate, probably because gypsum in the MSB system (10 wt% FGDG in B) was

Discussion
This study demonstrates the different As S/S performance of MST samples after four additives (Na 2 SO 4 , NaOH, Ca(NO 3 ) 2 and Ca(OH) 2 ) were added, as shown in Table S4. The MSB is selective for additives and only Ca(OH) 2 exerted a positive effect on the As S/S performance. The addition of Na 2 SO 4 had little effect on the pH values, UCS and As curing rate, probably because gypsum in the MSB system (10 wt% FGDG in B) was sufficient to provide sulphate for the hydration reaction. The addition of NaOH resulted in a pH value above 13, which inhibited the dissolution of Ca 2+ and hindered the hydration reaction, leading to a sharp decrease in UCS [30]. The amphoteric nature of arsenic in an alkaline environment and repulsion by OH − during the adsorption process of the hydration products led to the sharp decrease of the As curing rate [31,32]. Neither NaOH nor Ca(NO 3 ) 2 improved the As S/S performance, confirming that the individual addition of OH − without Ca 2+ would not improve As S/S efficiency and had a negative effect [33,34].
In addition, this study demonstrates that both the As(V) and As(III) S/S mechanism of MSB, and the effect of Ca(OH) 2 addition on As(V) and As(III) S/S are different. The addition of Ca(OH) 2 to MSB raised the pH of the reaction mixture (Figure 1), facilitated GGBFS dissolution and provided a source of Ca 2+ , thus improving the As(V) and As(III) S/S related to the hydration products. Adsorption or solid solution formation of As(V) and As(III) occurred due to the high specific surface area and amorphous properties of C-A-S-H, based on the shift in anti-symmetric Si-O(Al) stretching vibrations (v3) in the discussion above ( Figure 3) and SEM-EDS results (Figures 4 and 5).
Anion substitution in ettringite can occur via reaction with surface sites (ligand exchange), replacing Al/Ca coordinated surface OH − or substituting for sulphate inside channels (isomorphic substitution) [35]. This occurs as the ettringite surfaces are negatively charged under alkaline pH range conditions and due to the column and channel-like structure of ettringite, which is composed of columns of Ca 6 [36]. The extent of channel substitution of ettringite may be inversely proportional to the difference in size and electronegativity of the oxyanion compared with SO 4 2− (0.29 Å), resulting in As(V) (0.47 Å) being likelier to be channelsubstituted with sulphur than As(III) (0.69 Å) [37]. Thus, As(V) tended to form an innersphere complex in ettringite, whereas As(III) could form an outer-sphere complex [38], and the relatively larger size and charge of As(V) compared with SO 4 2− restricts the substitution inside channels without affecting the ettringite structure under high loading of As(V), which is consistent with the XRD, IR and SEM-EDS analysis results.
Previous studies [39][40][41] have verified the formation of Ca-As precipitates under alkaline conditions. In the present study, the distinct Ca-As(III) mineral phase characteristic peak was not found in the XRD patterns, maybe because of the content being below the detection limit of XRD analysis or mainly existing as an amorphous or more complex structure. When Ca(OH) 2 was added, Ca-As(V) changed from Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O (K sp = 10 −27.49 ) to Ca 5 (AsO 4 ) 3 (OH) (K sp = 10 −40.12 ) (Figure 2), which restricted As leaching [42].
Based on the findings of this study, alkaline solid waste with high calcium ion equilibrium concentrations and high alkalinity can be used to replace Ca(OH) 2 added to MSB for the S/S of high As-containing solid wastes for low carbon and environmental protection.

Conclusions
In this study, the effects of four additives (Na 2 SO 4 , NaOH, Ca(NO 3 ) 2 or Ca(OH) 2 ) on the As S/S performance of MST to enhance the As S/S efficiency were investigated. As-containing analytical reagents were used to prepare pure MSB paste to study the S/S mechanisms of As(III) and As(V) and the mechanism of efficiency improvement using Ca(OH) 2 , based on XRD, IR and SEM-EDS analysis. Based on the results obtained from this study, the following conclusions can be drawn:

•
An MSB is selective for the additives, and only Ca(OH) 2 exerts a positive effect on the S/S performance of MST. • As(V) can form an inner-sphere complex in ettringite, whereas As(III) can form an outer-sphere complex, and the relatively larger size and charge of As(V) compared with SO 4 2− restrict its substitution inside channels without affecting the ettringite structure under high loading of As(V).

•
The added Ca(OH) 2 facilitates the encapsulation and adsorption of As(V) and As(III) in MSB, and changes the Ca-As(V) from Ca 4 (OH) 2 (AsO 4 ) 2 ·4H 2 O (K sp = 10 −27.49 ) to Ca 5 (AsO 4 ) 3 (OH) (K sp = 10 −40.12 ). • Cement-free MSB with Ca(OH) 2 added is feasible to dispose of high As-containing solid wastes with high efficiency and a low carbon footprint.