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Industrial non-ferrous metal smelting and processing activities are a major contributor to the migration of arsenic-containing minerals throughout the environment, such as lollingite, which poses a severe threat to human health due to its rapid oxidation and subsequent release of arsenic (As). Herein, two phosphate mineral systems (potassium phosphate and calcium phosphate) were used to explore the potential for As solidification and establish the As immobilization mechanisms. Experimental results show that the compressive strength of the calcium phosphate system was higher than that of the potassium phosphate system due to the higher bond energy of the Ca–O–Mg–O–P–O system (31109 kJ / m o l), as compared to the K–O–Mg–O–P–O system (16189.2 kJ / m o l). Furthermore, the coordination capability of arsenic oxyanions with Ca or Fe ions was stronger than that of phosphorus oxyanions with Ca or Fe ions. The main mechanism of inhibition of As migration by magnesium phosphate cement (MPC) binders, was the formation of FeAsO 4.H 2 O, Ca 3 (AsO 4) 2.H 2 O, MgHAsO 4.H 2 O, CaMgAsO 4 (OH) and KMgAsO 4.6H 2 O coprecipitates, with physical encapsulation by MgKPO 4 ·6H 2 O (K-struvite), Ca 2 Mg(PO 4) 2 ·2H 2 O (Ca-struvite) and Mg 2 PO 4 (OH) phosphate-gel. Using Fe 2 O 3 -modified MPC binders with a Fe/As molar ratio of 3, the As immobilization efficiency reached 98.73%. The results of toxicity characteristic leaching procedure (TCLP) analysis confirmed that the maximum leached As concentrations from MPC binders cured for 7 days and 28 days were 0.31 mg/L and 0.06 mg/L, respectively, both of which were below the maximum permitted value for safe disposal (5 mg/L). These results verify the potential of MPC binders to be utilized as low-environmental impact and high-efficiency cementitious matrices for As pollution control and environmental remediation. [Display omitted] The highlights of our paper are. • The transformation and migration pathways of arsenic from lollingite to MPC binders was investigated. • Coprecipitation and physical encapsulation are the mechanisms to immobilize arsenic. • The coordination ability between As oxyanions and Ca or Fe ions, was stronger than that of phosphorus oxyanions. • The bond energy of the Ca–O–Mg–O–P–O system being stronger than that of the K–O–Mg–O–P–O system. • Fe 2 O 3 -modified MPC had excellent immobilization efficiency for As (99.9%). [ABSTRACT FROM AUTHOR]

The proper treatment of lollingite is of great significance due to its rapid oxidation leading to release of arsenic into the environment. Herein, a green multi-solid waste geopolymer, consisting of red mud, metakaolin, blast furnace slag, and flue gas desulfurization gypsum, was developed. The obtained red mud-metakaolin-based (RMM) geopolymer demonstrated good arsenic retention capability. The results showed that the replacement of SO 4 2− in ettringite with AsO 4 2− via ion exchange, formation of Ca-As and Fe-As precipitates, and physical encapsulation with aluminosilicate gel were the main mechanisms that prevented the release of arsenic. Further dissolution of ettringite in RMM was alleviated by adding a suitable amount of Ca(OH) 2 and controlling the pH of the leachate. TCLP results verified that RMM materials possessed an outstanding ability to stabilize arsenic, with a leaching rate below the permitted value of 5 mg/L for safe disposal. The low leachability of the RMM geopolymers (<0.50 mg/L) is potentially related to the pH buffering capacity of the hydration products at a pH range of 2–5. RMM geopolymers showed a high compressive strength (>15 MPa) and low arsenic leaching concentration (<2.66 mg/L) after 28 days of curing. These results demonstrate the potential of RMM geopolymers to be utilized as an environmentally friendly backfilling cementitious material for sustainable remediation of arsenic pollution. • The evolution pathways of arsenic from lollingite to RMM samples was investigated. • The dissolution of ettringite is correlated with the increase of As leachability. • Arsenic-bearing phases in RMM samples were demonstrated to be stable at pH 2–5. • RMM can be used as environmentally multi-solid waste cementitious material. [ABSTRACT FROM AUTHOR]