Phosphogypsum Leachates Research Articles

Effects of seawater mixing on the mobility of trace elements in acid phosphogypsum leachates

This research reports the effects of pH increase on contaminant mobility in phosphogypsum leachates by seawater mixing, as occurs with dumpings on marine environments. Acid leachates from a phosphogypsum stack located in the Estuary of Huelva (Spain) were mixed with seawater to achieve gradually pH7. Concentrations of Al, Fe, Cr, Pb and U in mixed solutions significantly decreased with increasing pH by sorption and/or precipitation processes. Nevertheless, this study provides insight into the high contribution of the phosphogypsum stack to the release of other toxic elements (Co, Ni, Cu, Zn, As, Cd and Sb) to the coastal areas, as 80–100% of their initial concentrations behaved conservatively in mixing solutions with no participation in sorption processes. Stable isotopes ruled out connexion between different phosphogypsum-related wastewaters and unveiled possible weathering inputs of estuarine waters to the stack. The urgency of adopting effective restoration measures in the study area is also stressed.

Impact Assessment of Phosphogypsum Leachate on Groundwater of Sfax-Agareb (Southeast of Tunisia): Using Geochemical and Isotopic Investigation

The production of phosphoric acid by the Tunisian Chemical Group, in Sfax, Tunisia, led to the degradation of the groundwater quality of the Sfax-Agareb aquifer mainly by the phosphogypsum leachates infiltration. Spatiotemporal monitoring of the quality of groundwater was carried out by performing bimonthly sampling between October 2013 and October 2014. Samples culled in the current study were subject to physicochemical parameters measurements and analysis of the major elements, orthophosphates, fluorine, trace metals, and stable isotopes (18O,2H). The obtained results show that the phosphogypsum leachates infiltration has a major effect on the downstream part of the aquifer, where the highest values of conductivity,SO42-, Ortho-P, andF-, and the lowest pH were recorded. In addition, these results indicated that phosphogypsum leachates contained much higher amount of Cr, Cd, Zn, Cu, Fe, and Al compared to the groundwater. Spatiotemporal variation of the conductivity and concentrations of major elements is linked to the phosphogypsum leachates infiltration as well as to a wide range of factors such as the natural conditions of feeding and the water residence time. Contents ofO18and2H showed that the water of the Sfax-Agareb aquifer undergoes a large scale evaporation process originated from recent rainfall.

Applying physicochemical approaches to control phosphogypsum heavy metal releases in aquatic environment

One of the most important sources of solid waste in the Mediterranean Basin ecosystem originated from the phosphate fertilizer industries, which discharge phosphogypsum (PG) directly into aquatic environments or are stacked on stockpiles. The present study investigates metal release from PG under the influence of variable pH, increasing PG mass content, and complexing organic matter ligands. Major ions from PG leachates, grain size and charge, main functional groups along with metal leachability (Pb, Cd, Cr, Cu, and Zn) were determined using ion chromatography, laser diffraction, zetameter, Fourier transform infrared spectroscopy, and atomic absorption spectroscopy, respectively. The complete dissolution of PG recorded is at 2 g/L. Saturation and supersaturation with respect to PG may occur at concentrations of 3 and 4 g/L, respectively, revealing a clustering phenomenon leading to heavy metal encapsulation within the aggregates. Organic ligands such as citrate may trigger the cationic exchange within the PG suspension leading to ion release. As these factors are considered as specific process involving the release of contaminants from PG during storage under natural conditions, this study could set the foundations for PG remediation in aquatic environment. Organic ligands under controlled pH conditions could be utilized in treating fertilizer industrial wastes by taking into consideration the particularity of the receiving area, thus decreasing metal hazardous impact on natural media.

Phosphogypsum Leachate: Treatment Feasibility in a Membrane Plant

In landfills for phosphogypsum disposal, a large volume of leachate (PGL) wastewater generated by the residual process water and by the rainfall percolation through the solid material must be managed. The leachate shall be properly treated before its discharge into surface water or its reuse; it has a very low pH and high concentrations of hazardous contaminants such as fluoride, nitrate, phosphate, ammonia, and heavy metals. On the basis of a technical evaluation, a membrane filtration process for the treatment of the leachate was proposed to produce a very good quality permeate, in compliance with the standard limit (CEE 91/271 directive). A membrane pilot plant, composed of an initial nanofiltration (NF) treatment and two reverse osmosis membrane stages placed in series (RO1 and RO2), was employed to produce a permeate that can be discharged or reused. A first campaign was carried out for comparison using the membrane pilot plant in the basic configuration; in the NF step, a recovery of 58% was obtaine...

Use of combined solvent extraction-electrodeposition techniques to uranium and thorium isotopes analysis in industrial phosphoric acid, phosphate and phosphogypsum by alpha-particle spectrometry

Summary A method combining liquid-liquid extraction and electrodeposition procedures is carried out to determine isotopic composition of uranium and thorium in technical wet phosphoric acid, phosphate rock and phosphogypsum leachates, by α-particle spectrometry. Extraction experiments are performed with diethylether from 5.57 M calcium nitrate solution with nitric acid concentration of 2 M. Uranium is extracted in the pH range 0.6-0.8 and thorium is extracted after acidification with 2 M HNO3. Alpha-sources are obtained by a selective electrodeposition of U(VI) or Th(IV) directly from the extracting solution without applying back-extraction or purification steps. The dependence of the electrodeposition and the overall recovery of U and/or Th are examined as a function of the amount of phosphate ions (C PO₄³⁻) in the electrolyte and of the aqueous phase, respectively. In all cases a decrease of the deposition yield with C PO₄³⁻ is observed. An improvement in the uranium overall recovery is achieved from 7.57 to 9.57 M nitrate solution and PO4 3- concentration lower than 0.8 M. The method proposed is applied to technical wet phosphoric acid, phosphate rock and phosphogypsum leachates. Recoveries of 74%-93% are obtained for uranium and 54%-69% for thorium. Good resolutions (37-52 keV) are also achieved under optimum conditions. These resolutions allow to make accurate determination of U and Th isotopes in the above mentioned samples.

Fluoride sorption and mineral stability in an Alberta soil interacting with phosphogypsum leachate

A by-product of phosphate fertilizer production is phosphogypsum (PG). Relatively high F concentrations occur in the solid phase and in the highly acidic process water (PW) that initially fills its pores. The present research addresses the fate of F and the effects on mineral stability as phosphogypsum leachate (PGL) interacts with calcareous (Ck) soil and non-calcareous (Bt) soil. In a sequential batch equilibration, soils were exposed to fresh PGL during 30 d-long extractions. Major ions, pH and electrical conductivity (EC) were monitored. At 28 d, F concentration decreased from 31 mg L−1 in the PGL to 11 mg L−1 in the Bt-PGL mixture and 22 mg L−1 in the Ck-PGL mixture. Saturation indices (SIs) for fluorite ranged from 2.1 to −3.6. Extracts were closer to saturation with fluorite than with the other F minerals studied. Using alternative geochemical constants, fluorapatite was predicted to be stable in Bt-PGL extracts. The day 28 Bt-PGL extract was also near equilibrium with alunite, basaluminite, jurbanite, gypsum, amorphous silica, and quartz. Of the phyllosilicates in the soil-PGL mixtures at day 28, only kaolinite was predicted to be stable in only the Bt-PGL mixture. The potential for clay mineral instability and accelerated dissolution in F-rich PGL needs to be considered m the design of PG repositories. Key words: Fluoride, phosphogypsum, waste management, leachates, geochemical modeling, mineral stability

Weathering of clay minerals induced by fluoride-containing solutions from phosphogypsum by-product

The production of phosphate fertilizer from phosphate rock results in the formation of phosphogypsum (PG) and HF. Soluble F− ends up in process water and in the pore space of the PG by-product. This study determined changes in the amount, properties and mineral composition of soil clay upon reaction with acidic PG solutions. Calcareous and non calcareous soils were reacted with PG leachates (PG-L, 30 mg F− L−1) and process water (PG-PW, 6070 mg F− L−1) using a sequential batch equilibration method. Phosphogypsum-L did not alter the clay fraction as much as PG-PW, which dissolved as much as two-thirds of the clay fraction. Phosphogypsum-PW reduced surface area of clay fractions from about 500 to 150 m2 g−1; Cation exchange capacity CEC was decreased from about 60 to 17 cmolc kg−1 clay. Reduction in surface area and CEC in PG-PW treated soil corresponded to near complete dissolution of smectite and a negative enrichment of mica. Kaolinite in coarse clay of the PG-PW treatment was reduced to 20% of the control and was completely destroyed in fine clay fractions. Amorphous silica was produced from the F−-induced dissolution of clay. The results indicate there is likely to be considerable alteration of minerals in subsoils beneath PG waste repositories and of clay liners used at some storage facilities. Key words: Clay liners, landfills, dissolution, waste management, leachates, geochemical reactions, mineral stability

Phosphogypsum Waste Anion Removal by Soil Minerals

The removal of F-, PO4-3, and SO4-2 from phosphogypsum leachate (pH 1.7) by soil minerals was investigated using a serial batch procedure. PH was the controlling factor in removal of orthophosphate and fluoride, but had less effect on sulfate. Calcium carbonate precipitated major fractions of fluoride and orthophosphate, and smaller amounts of sulfate, during the early stages of leaching; with continued leaching, redissolution of fluoride and orthophosphate occurred, resulting in release of these anions at concentrations exceeding those in the original phosphogypsum leachate. Removal of fluoride and orthophosphate by kaolinite and montmorillonite was strongly affected by pH, due to complex formation, degree of ionization, and charge on the clays. Iron (III) hydroxide had little effect on fluoride or orthophosphate removal due to formation of Ca‐, Al‐, and Fe‐complexes which possibly interfered with adsorption. The iron hydroxide was important to sulfate removal at all pH values; the removal was markedly e...