Dynamics of metals in backfill of a phosphate mine of guiyang, China using a three-step sequential extraction technique

Remediation of cadmium-contaminated soil by micro-nano nitrogen-doped biochar and its mechanisms

Cadmium-contaminated soils are an urgent problem that needs to be solved in many countries and regions. In this study, a new heavy metal passivator, micro-nano nitrogen-doped biochar (Nm-NBC), was prepared by introducing nitrogen into biochar. Soybean was used as an experimental plant to compare the effects of corn straw biochar (CBC, not modified), ammonium chloride modified corn straw biochar (NBC), and micro-nano nitrogen-doped biochar (Nm-NBC) on the remediation of Cdcontaminated soil. The results showed that the biomass of soybean, pH, organic matter, and total nitrogen content of the Cd-contaminated soil significantly increased, and the available Cd in soil significantly reduced (P < 0.05) when CBC, NBC, and Nm-NBC were added. The effect was as follows: Nm-NBC > NBC > CBC; Nm-NBC had the best result. When 1% Nm-NBC added to the soil, the Cd content in beans reduced by 68.09%. BET, FTIR, XPS, and SEM were used to analyze the characteristics of Nm-NBC and its mechanisms in the remediation of Cd-contaminated soils. The results showed that Nm-NBC had larger specific surface area and abundant functional groups; -COOH and graphitic nitrogen in Nm-NBC can form Cd-O bond and Cd-π with Cd(II) in the soil. Therefore, Nm-NBC prepared by introducing nitrogen into biochar has a promising application in the remediation of Cd-contaminated soil.

Preparation and performance of composite activated slag-based binder for cemented paste backfill

The utilization of blast furnace slag (BFS) and fly ash (FA) to replace ordinary portland cement (OPC) has become a hot topic in the preparation of low-cost cemented paste backfill (CPB). This study has prepared a composite activated slag-based binder (CASB) using BFS and FA as the basic raw materials and desulfurization gypsum (DG) and cement clinker (CC) as the activator. The optimum ratio of CASB was determined based on the orthogonal test and the efficacy coefficient method. The hydration products and hydration mechanism of CASB materials were further investigated using XRD, TG, and SEM tests; on this basis, the compressive strength of hardened CASB-CPB under different working conditions and the rheological properties of fresh slurry were investigated, and the cost analysis and environmental effects of CASB were carried out. The results show that the optimum ratio of CASB was 15:12:13:60 for FA: CC: DG: BFS; the hydration mechanism of CASB was the coupled alkali-sulfate activation of CC and DG, and the main hydration products were hydrated calcium silicate gels (C-S-H gels) and ettringite (AFt); increasing the mass concentration (Cw) at a constant cement-aggregate ratio (C/A), which caused a significant improvement in the compressive strength at 7 and 28 d while reduced the flowability of the slurry; CASB considerably reduced the filling cost compared to OPC, and effectively immobilization the heavy metals in the tailings. This paper has developed a cement alternative binder of CASB, which has considerable significance for the comprehensive utilization of solid waste, reduction of filling costs, and improvement of economic and ecological benefits of the mine.

Using Iron Tailings for Phosphate Removal in Cemented Phosphogypsum (PG) Backfill

Compared with the post-treatment of pollutants, such as the removal of phosphate from wastewater, it is more important to develop effective emission control strategies to reduce phosphate pollution. Phosphogypsum (PG) is a typical solid waste byproduct of phosphate production and contains high amounts of residual phosphate. In order to control the phosphate emissions during the recycling of PG aggregates for cemented backfill, another solid waste product—iron tailings (ITs)—was added during the preparation of backfill slurry. The results showed that the ITs effectively accelerated the phosphate removal in cemented PG backfill, enabling the quick reduction in the phosphate concentration to the discharge standard (<0.5 mg/L) within 15 min. This means that the emissions of phosphate to bleeding water were effectively controlled. The adsorption experiment showed that phosphate was adsorbed by the ITs, and the adsorption data fitted well with the Langmuir adsorption model (R2 = 0.98) and pseudo-second-order kinetic model (R2 = 0.99), indicating that the phosphate adsorption of ITs was a monolayer chemical adsorption. Furthermore, an unconfined compressive strength (UCS) test was performed on the backfill with the addition of ITs. Compared to the control group (without ITs), the UCS of backfill with 20% ITs increased from 1.08 MPa to 1.33 MPa, indicating that the addition of solid waste could be beneficial to the strength development of the backfill by mitigating the interference of phosphate with the hydration process. The backfill cured for 28 d was selected for the toxic leaching test, and the phosphate concentration in the leachates was always below 0.02 mg/L, indicating that ITs can effectively immobilize phosphate in backfill for a long time.

The Release of Pollutants through the Bleeding of Cemented Phosphogypsum Backfill: Link to Protocols for Slurry Preparation

The present study investigated the effects of protocols for slurry preparation on the release of pollutants into bleeding water from cemented phosphogypsum (PG) backfill. Backfill slurry was prepared using four different protocols in which different parameters varied, including binder/PG ratio, solid concentration, binder type and mixing procedure. The concentrations of phosphate, fluoride and sulfate and the pH values of the obtained bleeding water were measured. The results demonstrated that the slurry preparation protocols affected the quantities of pollutants through the concentrations of pollutants in bleeding water and the bleeding rate. On the one hand, the binder/PG ratio was the key factor influencing the concentrations of all pollutants in bleeding water. Comparatively speaking, the binder type and mixing procedure had an obvious influence on the fluoride concentration but had little influence on the phosphate and sulfate concentrations in the bleeding water. On the other hand, the protocols for slurry preparation affected the bleeding rate by determining the water retention and water content of the backfill slurry. The most effective protocol for slurry preparation for cemented PG backfill could reduce the bleeding rate and enhance the immobilization of pollutants, minimizing the phosphate concentration in bleeding water to below 0.2 mg/L. However, it appeared that the fluoride concentration was still tens of milligrams per liter (over the limit of 10 ten milligrams per liter), to which attention should be paid.

Structural study and metal speciation assessments of waste PCBs and environmental implications: Outlooks for choosing efficient recycling routes

Environmental protection from risks and disposal management of discarded mobile phone printed circuit boards (MPhPCBs) is a global issue. Although recycling is proposed as a solution, it is challenging to choose a sustainable method due to the insufficient recognition from extreme structural heterogeneity of these wastes based on their types. To this end, a thorough study on the structural characterization of PCBs using different analyses and metal speciation by sequential extraction procedure were performed. Understanding these information is an essential step in order to choose efficient methods to maximize selective recycling of metals and minimize environmental implications. PCBs were found to be potent metallic reservoirs after all metal content of PCBs were precisely measured. The structural analysis results of the sample included identification of different phases, functional groups, 45.1 % of the crystalline and 54.9 % of amorphous, the mesoporous nature (pore diameter mean ∼ 7.24 nm), hydrophobic property (contact angle ∼93.4°), the positive ζ-potential of particles at pH < (isoelectric point ∼5.4) and vice versa, and the particle size that were not oversized. The metal speciation outcome indicated over 80 % of the total content of elements such as Si, Sn, Ag, Au, Sr, Al, Cr, Nd, Ca, Ba, and P was in a solid crystal structure/residual fraction, which were hard recycled. The assessment of contamination levels of waste indicated the considerable contamination for the environment at global contamination factor ∼27.7, the moderate ecological risk at potential ecological risk assessment ∼82.9, and threats to public health. In addition, the metals of Pr, Mn, and Zn pose high risks because of their risk assessment code values of 42.7 %, 36.7 %, and 33.1 %, respectively. Leaching tests proved Waste Extraction Test was an aggressive method. ANC4 proposed high level of H⁺ consumption are required for metal leaching in future works.

Effect of phosphorus on the properties of phosphogypsum-based cemented backfill

When phosphogypsum (PG) is used as an aggregate for backfill, phosphate in the PG might influence the hydration process and escape into the environment. The current study aimed to understand phosphate dynamics during the PG-based backfilling process by adding different amounts and types of phosphates (H₃PO₄, KH₂PO₄, K₃PO_4, and Ca₃(PO₄)₂). The results indicate that the majority of the phosphate was first immobilized by PG depending on the types, and the residual dissolved phosphate (RDP) could be further stabilized/solidified (S/S) in the backfill via the hydration process. However, increasing RDP content lowered unconfined compressive strength of the backfill, attributing to the suppression of the hydration process and a loosened backfill structure. Furthermore, the environmental behavior of phosphate was studied by measuring dissolved phosphate in bleeding water and leachate. For bleeding water, a high RDP content might lead to the phosphate concentration exceeding the national standard limit (GB 8978-1996) depending on the phosphate types, and it was recommended that the RDP content should be controlled or converted to Ca₃(PO₄)₂ or K₃PO₄ before PG inclusion into in the backfill. For leachate, the phosphate concentration was always below the standard limit, indicating that the cemented backfill ensured long-term S/S of the phosphate.

Triaxial compression behaviour of gangue solid wastes under effects of particle size and confining pressure

Underground coal mining leads to environmental problems such as gangue pollution, surface subsidence and soil erosion, etc. Solid backfilling coal mining (SBCM) can control the strata movement, reducing gangue discharge and environmental pollution in mining areas. Gangue solid wastes (GSW) are backfilled into the goaf space as the supports for the overburden strata in solid backfilling coal mining. In this case, GSW are under triaxial compression. The deformation characteristics of the GSW under this loading condition are the key factors determining the control efficiency of strata movement and surface subsidence. The influence of the particle size grade and the confining pressure condition on the deformation and breakage characteristics of the GSW is studied based on large-scale triaxial compression tests in this paper. Also, the effect of particle breakage on the deformation of the GSW is revealed. The results indicate that in the triaxial compression process, the GSW exhibits strain hardening characteristics and volume decrease under compressive loading. The maximum load-bearing stress is more sensitive to the confining pressure condition but less sensitive to the particle size grade. The maximum load-bearing stress increases linearly with the confining pressure. Particle breakage of the GSW is notably influenced by the particle size grade but almost independent of the confining pressure condition. The S1 specimen with a reasonable proportion has the minimum relative breakage index, while the S3 specimen that mainly contains large particles has the maximum relative breakage index. The research outcomes are significant to the understanding of the mechanical characteristics of GSW, the selection and preparation of the backfilling materials in the field practices and the ensurence of the backfilling efficiency.

Effects of acid and phosphate on arsenic solidification in a phosphogypsum-based cement backfill process

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects. Cement backfill technology is an effective management technique that is used to store PG to prevent such problems. The goal of this paper is to study the influencing factors and mechanism of arsenic stabilization in a PG-based cement backfill process. First, a leaching toxicity test was conducted, which showed that the arsenic concentration in PG batches ranged from 129.1 μg L⁻¹ to 407.1 μg L⁻¹, which were all far above the standard limit (10 μg L⁻¹) set by GB/T 14848-93. In addition, the arsenic content was higher in samples with larger PG particles. Secondly, hydrogen and phosphate ions were added to the backfill to investigate how they influenced arsenic solidification, and the results indicated that phosphate ions, rather than hydrogen ions, delayed the arsenic solidification process. This suggests that controlling the soluble phosphate in PG will help reduce arsenic pollution during backfilling. A toxicity leaching test was carried out after backfill samples were cured for 28 d. All arsenic concentrations were below the standard limit, indicating that the cement backfill technology ensured the long-term solidification and stabilization of arsenic.

Phosphogypsum (PG) produced during phosphoric acid production contains significant amounts of arsenic and can potentially cause adverse environmental and health effects.

Durability Evaluation of Phosphogypsum-Based Cemented Backfill Through Drying-Wetting Cycles

In this study, the durability of phosphogypsum (PG)-based cemented backfill was investigated by drying-wetting cycles to explore deterioration of its strength and the release of impurities. The leachates in this test were composed of deionized water, 5% Na2SO4 solution, 5% NaCl solution, and a range of sulfuric acid solutions with pH values of 1.5, 3, and 5. After drying-wetting cycles, unconfined compressive strength (UCS), visual deterioration, porosity, microstructure and concentrations of phosphate and fluoride in the leachates were measured. The results showed that both saline and acidic solutions could lead to strength reduction of PG-based cemented backfill under different deterioration mechanisms. The mechanical damage of salinity was caused by micro-cracking and degradation of C–S–H. However, the H+ broke the backfill by dissolving hydration products, leaving the conjunctures between PG particles weakened. Furthermore, the environmental impact was investigated by measuring the concentration of phosphate and fluoride in the leachates. In acidic solutions, the release of phosphate and fluoride was greatly enhanced by H+. Compared to the great strength deterioration in saline leachates, the concentration of phosphate and fluoride were similar to that of deionized water, indicating that saline solutions had little impact on the release of hazardous impurities.

Slurry Preparation Effects on the Cemented Phosphogypsum Backfill through an Orthogonal Experiment

The cemented phosphogypsum (PG) backfill technique provides a new method for massive consumption of PG, and therefore alleviating the environmental pollution of PG. This study considered the effects of slurry preparation on the performance of cemented PG backfill. A L16(44) orthogonal experiment was designed to analyze four factors, namely the solid content, phosphogypsum-to-binder ratio (PG/B ratio), stirring time and stirring speed, with each factor having four levels. According to the range analysis, the solid content played the dominant role in controlling the bleeding rate, while the setting times strongly depended on the PG/B ratio. In terms of strength development of the backfill, the PG/B ratio was shown to be the most significant factor determining the unconfined compressive strength (UCS), followed by the solid content, stirring time and stirring speed. Furthermore, the results showed that the slurry preparation affected the environmental behavior of impurities that originated in PG. By analyzing the concentrations of impurities in the bleeding water of the slurry as well as the leachates of the tank leaching test, the results showed that the release of F− and SO42− was aggravated clearly with the increase in the PG/B ratio, while the release of PO43− always remained at relatively low levels.