The aim of this study was to improve the immobilization capacity of a binder prepared from As-containing biohydrometallurgy waste (BAW) on arsenic (As) by modifying it with ferric/ferrous salts. The study investigated the chemical fractions of As and its leaching characteristics. Results indicated that the addition of ferric/ferrous salts significantly reduced the mid acido-soluble fraction of As, resulting in decreased As leaching. Among the four ferric/ferrous salts studied, ferrous sulfate (FeS2) performed the best, with only 0.5 wt% of FeS2 achieving satisfactory results. The influence of ferric/ferrous salts on the strength development and reaction products were also investigated. Although the incorporation of ferric/ferrous salts reduced As leaching, it also decreased the early age compressive strength of BAW. Ferric salts had a more significant impact on the strength evolution. In addition, the mechanism of Fe species on As stability using density function theory calculation was explored. Results suggested that As species could be adsorbed on the surface of Fe species through strong covalent bonds. Overall, the study demonstrated that modifying BAW using ferric/ferrous salts could effectively reduce As leaching. However, the decrease in strength especially at the early ages must be taken into consideration when using this method.

The aim of this study was to improve the immobilization capacity of a binder prepared from As-containing biohydrometallurgy waste (BAW) on arsenic (As) by modifying it with halloysite nanotubes (HNTs) and biochar (BC). The study investigated the influence of HNTs and BC on the chemical fractions and leaching characteristics of As, as well as the influence on the compressive strength of BAW. The results indicated that the addition of HNTs and BC effectively decreased As leaching. The presence of 1.0 wt% HNTs decreased the As leaching concentration from 1.08 mg/L to 0.15 mg/L, with the corresponding immobilization rate reaching about 90.9 %. A high amount of BC seemed to show better performance in improving the As immobilization capacity of BAW. However, a strongly reduced early compressive strength of BAW was observed, making it unsuitable to be used as an additive in this situation. The effects of HNTs on the increase of As immobilization capacity of BAW were attributed to two aspects. Firstly, As species were adsorbed onto the surface of HNTs via H-bonds, which was verified via density functional theory calculation. Secondly, the addition of HNTs decreased the pore volume of BAW, leading to a more compact structure, and hence increasing the physical encapsulation capacity for As. Environmental implication: The rational disposal of arsenic-containing biohydrometallurgy waste has always been a top priority for the green and low-carbon development of the metallurgical industry. In this article, we have taken the perspective of large-scale resource utilization of solid waste and pollution control, and developed arsenic-containing biohydrometallurgy waste into a cementitious material, and enhancing arsenic immobilization capacity with the addition of HNTs and BC. This study provides an effective method for the rational disposal of arsenic-containing biohydrometallurgy waste.

Hazardous wastes are commonly immobilized using cementitious materials. Ettringite (Ca₆Al₂(OH)₁₂(SO₄)₃∙26H₂O) is one of the main components of hydrated cement and is known to stabilize anionic species. Thus, it is a promising material for the stabilization of arsenate (As(V)) in cementitious waste streams. However, the effects of As(V) incorporation into the ettringite structure on the structural properties of As and ettringite remain unclear. Herein, As(V) doping into the structure of ettringite was investigated based on the first-principles calculations using density functional theory (DFT), and the results suggested that the mechanism involving the substitution of AsO₄ ³⁻ by SO₄ ²⁻ was energetically favorable and that As doping increased the total bond order of ettringite, leading to a more stable structure. The elastic properties and anisotropy of ettringite and As-doped ettringite were also investigated using DFT calculations. The results showed that the mechanical properties of the As-doped ettringite structure were stable according to Born's criterion. Moreover, As doping increased the elastic constants and elastic modulus of ettringite but reduced its elastic anisotropy. These results provide valuable insights into the long-term stabilization of As species and their migration in cementitious wastes.