Immobilization of heavy metals and radionuclides of phosphogypsum origin in anhydrite cement matrix and verification of environmental safety by leaching and spectroscopy methods

Abstract

Under the conditions of increasing accumulation of phosphogypsum as a large-tonnage industrial waste, the aim of this study was to develop an anhydrite binder based on thermoactivated phosphogypsum and to assess the efficiency of immobilization of heavy metals and natural radionuclides in its matrix with verification of environmental safety. To obtain anhydrite cement, phosphogypsum was calcined at 700-900 °C, followed by grinding and modification with CaO and K2SO4; the structure and phase composition of the composites were studied by XRD, SEM/EDS, and mercury porosimetry; radiation characteristics were analyzed by gamma spectrometry; leachability of lead, cadmium, arsenic, and zinc was determined by dynamic and static tests using atomic absorption spectrometry; physical and mechanical properties were measured by standard tests of 40×40×160 mm samples, with statistical processing performed according to the experimental design. It was established that the comprehensively modified composition B forms a densified structure with a density of about 2100 kg/m³, compressive strength of 34,15 MPa, softening coefficient of 0.81, and reduced open porosity (7,45%), ensuring decreased diffusion permeability. The effective specific activity of natural radionuclides for the optimal composition (Aeff ≈ 406 Bq/kg) corresponds to class II materials and allows their use in industrial and road–civil construction. The concentrations of Pb, Cd, As and Zn in the aqueous extract are significantly below permissible limits, the immobilization degree reaches 78-87%, and residual toxicant contents do not pose environmental risks during operation. A nonlinear inverse relationship was found between open porosity and immobilization degree, confirming the dominance of the diffusion-barrier retention mechanism of pollutants in the anhydrite matrix. It is discussed that the use of modified phosphogypsum in anhydrite binders makes it possible to simultaneously address the issues of hazardous waste utilization, production of medium-strength construction materials, and reduction of anthropogenic load in regions with developed chemical industry.