Introduction

Phosphorus is essential to all life. It is also a limited natural resource, the cycling of which is accelerated by human activity due to its use as fertilizer. With diminishing geologic P reserves, and an ever increasing global population, P lost in the human cycle needs to be recouped [1]. The biomineral struvite (MgNH4PO4·6H2O; MAP) can help to close the P cycle as MAP recovered from human and animal wastes and wastewater has been targeted as an alternate, sustainable source of fertilizer [2–4]. This takes advantage of the spontaneous precipitation of MAP during some wastewater treatment processes [4–7], the ability to oversaturate and precipitate MAP from human urine [8, 9], and the formation of MAP from sewage sludge and animal waste during biofuels production [4, 10]. The recovery and repurposing of MAP from these sources would utilize P that is already a part of the human cycle, which is otherwise lost to the environment, often resulting in pollution. This in turn would encourage sustainable use of P, and aid in the conservation of non-renewable geologic P reserves [11].

Various wastes and wastewaters contain several contaminants. These include the metalloid chromium (Cr), which has been detected in MAP precipitated from both sewage sludge effluent and urine [12, 13]. This is concerning as Cr can displace and follow the same pathways as P in both geochemical and biological systems, with toxic effects for plants and organisms. The solid reactivity and toxicity of Cr is contingent upon oxidation state, with Cr(III) relatively immobile in the presence of solids and less toxic compared to Cr(VI), which remains in solution and is easily transported [14]. Sorption of Cr with MAP may proceed by several mechanisms including substitution into the MAP mineral structure during crystallization, adsorption to already precipitated MAP, or by precipitation of oversaturated Cr phases. These processes can alter the composition of MAP, influencing both the recovery process and the subsequent decomposition and release of nutrients. The total Cr content of MAP also requires consideration if this product is to be introduced to agricultural soils and crops.

Thermal analysis can be a powerful tool in the characterization of MAP, as structural water (H2O(s)) and ammonium ions (NH4 +) are readily evolved on heating. Previous studies using thermogravimetric analysis (TG) and differential thermal analysis (DTA) revealed endothermic transitions at ~80–250 °C associated with the progressive loss of volatiles from MAP [15–18]. Thermal analysis may also be useful in determining the effect of sorbed contaminants on mineral properties. For example, TG revealed increased mass loss for Cr(III)-substituted hydroxylapatite (Ca5(PO4)3OH; HAP) due to incorporation of additional hydroxyl ions with Cr [19]. Thus, this is a viable approach for studying metal sorption, however, heretofore there are no documented thermoanalytical studies of Cr sorption, or that of any other metals, with MAP, due to limited prior geochemical interest in this mineral.

The current study combines TG and differential scanning calorimetry (DSC) with evolved gas analysis by Fourier transform infrared spectroscopy (FT-IR) to study Cr sorbed MAP. The hyphenated technique has been used to characterize a wide variety of materials [20–23]. For MAP this approach is effective as FT-IR can distinguish between evolved H2O(g) and NH3(g), which have similar molecular masses and cannot be identified by TG alone. Experiments were designed to precipitate MAP in the presence of either Cr(III) or Cr(VI) over a range of initial Cr concentrations. Results from TG/DSC–FT-IR were used to assess the effect of Cr on the both the stability and composition of MAP. Furthermore, the technique was used to propose potential mechanisms of Cr sorption with MAP. Results from this study demonstrate that Cr can contaminate MAP fertilizers, influencing the release, and thus cycling of P and contaminants in agricultural systems.

Materials and methods

Precipitation of MAP in the presence of Cr

Experiments were designed to assess the sorption of Cr with actively precipitating MAP. The required solution chemistry was calculated using the program PHREEQC [24] with the minteq.v4 database and MAP solubility values from the literature [25]. An initial MAP saturation index of 2 (log Ω MAP = 2) was used as this is known to result in the formation of a homogeneous MAP precipitate [26]. Stock solutions of MgCl2·6H2O and (NH4)2HPO4 were prepared at concentrations so that when mixed, the desired MAP saturation would be attained. Prior to mixing either the Mg solution was spiked with Cr(III) as Cr(NO3)3·9H2O or the PO4 solution with Cr(VI) as Na2CrO4 for final Cr concentrations of 1–100 μM. Solutions were then reacted in the presence of 0.1 g L−1 MAP seed, and spontaneous precipitation allowed to proceed at pH 7.8 ± 0.2. All samples were prepared in duplicate. Blanks, without added Cr (0 μM), were included for each experiment. After 7 days, reacted solids were recovered by filtration and allowed to air dry for subsequent characterization. An aliquot of filtered solution was retained, and a small portion of solid was acid digested to determine final aqueous and solid Cr concentration, respectively, by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Perkin Elmer Optima DV5300 instrument.

X-ray diffraction

Selected solids were characterized by X-ray diffraction (XRD) using a Phillips XPert Pro instrument with a CuKα radiation source. Data were collected over a range of 5°–60° 2θ in 0.02 step sizes with integration times of 0.5 s. The experimental diffraction pattern for synthesized MAP was compared to the theoretical pattern derived from the literature [27].

Thermal analysis

All recovered solids were subject to TG/DSC–FT-IR analysis. For the hyphenated technique, a simultaneous thermal analyzer, Perkin Elmer STA 6000, was connected to a Spectrum 100 FT-IR using a gas transfer line and specially designed FT-IR cell. Approximately 30 mg of sample was placed in a ceramic crucible and heated at a rate of 10 °C min−1 over a range of 25–400 °C. The sample purge was dry N2(g) with a flow rate of 20 mL min−1. Evolved gases were transported through the transfer line, into the FT-IR cell, both of which were heated to 250 °C to prevent gas condensation. The FT-IR was configured to continuously collect background-corrected spectra over a wavenumber range of 600–4,000 cm−1 for the duration of the temperature program.

Results and discussion

Sorption of Cr by MAP

Results from ICP-OES analysis of aqueous and solid samples indicate notable differences in the affinity of Cr for MAP contingent upon oxidation state (Fig. 1). Removal of both aqueous Cr(III) and Cr(VI) increases with initial concentration. The minimum final aqueous concentrations are 1 and 83% of the initial Cr(III) and Cr(VI), respectively (Fig. 1a). Commensurate with the trend for aqueous samples, the solid loadings of Cr(III), and more subtly, that of Cr(VI) increase with initial concentration (Fig. 1b). Significantly higher solid loadings for Cr(III) compared with Cr(VI), coupled with results from aqueous samples, indicates greater affinity of Cr(III) for MAP.

A high sorption affinity of Cr(III) for MAP, also compared to that for other phosphate minerals including HAP, has been observed previously [28]. For Cr(VI) samples, low loadings may be a result of competition between the dominant CrO4 2−(aq) and HPO4 2−(aq) species in solution for similar MAP sorption sites. In addition, sorption of CrO4 2−(aq) may be inhibited by the predominantly negative surface charge of precipitating MAP at the reaction pH 7.8 ± 0.2 [29]. The highest Cr(III) solid concentrations may be due to precipitation of amorphous solids, such as Cr(OH)3(am), which may accumulate at the MAP surface.

Fig. 1
figure 1

Sorption of Cr(III) and Cr(VI) with MAP after 7 days reaction a percentage Cr remaining in solution and b final loading of Cr on the solid

Full size image

X-ray diffraction

Theoretical XRD patterns for MAP are quite variable depending on the geological or biological source, or method of synthesis. The highest intensity peaks at 14.5° and 30.5° 2θ are characteristic of MAP, and evident in the XRD pattern of the solid synthesized in the current study (Fig. 2). This confirms the formation of MAP using the chosen method of synthesis.

Fig. 2
figure 2

XRD pattern for MAP synthesized in this study and diffraction lines for struvite from Ferraris et al. [27]

Full size image

Thermal analysis of unreacted MAP

For unreacted MAP, DSC reveals an endothermic peak at 126.1 ± 0.5 °C with an onset temperature of 87 ± 0.5 °C (Fig. 3a). The transition is associated with significant mass loss from the sample as indicated by TG, and an increase in the absorbance in the FT-IR (Fig. 3a). The temperature-dependent FT-IR spectra reveal changes in absorbance in the 1,300–2,000 cm−1 range, and at >3,500 cm−1, characteristic of H2O(g), and at ~1,000 cm−1, due to NH3(g) (Fig. 3b). Release of these gases is maximized near the endothermic transition, with continued release up to 200 °C. Additional NH3(g) is evolved up to 300 °C. At the end of the heating program at 400 °C all volatiles are released from the sample with an overall mass loss of 52.9%. For the theoretical MAP composition, structural H2O and NH4 + account for 50.9% of the mass. Thus, under the current conditions of MAP synthesis, a small fraction of additional volatiles appears to be associated with the structure.

Fig. 3
figure 3

TG/DSC–FT-IR results for unreacted MAP. a DSC heat flow, percentage mass loss by TG and absorbance in the FT-IR and b temperature-dependent FT-IR spectra

Full size image

Previous thermoanalytical studies indicated a multi-stage decomposition for MAP. For synthetic MAP, endothermic events at ~103 and 235 °C, and an exotherm at ~600 °C were associated with decomposition to the monohydrate (NH4MgPO4·H2O), formation of an amorphous phase (MgHPO4), and formation of the pyrophosphate (Mg2P2O7) [15, 16]. Renal stones composed of MAP were also found to undergo endothermic and exothermic reactions within a range of 80–250 °C and 650–700 °C, respectively [17]. In the current study, the temperature range of MAP decomposition from the onset of the endothermic transition is ~87–300 °C, consistent with results for both synthetic and biogenic MAP. Slight differences in the temperature of the primary endothermic peak and the absence of a second distinct endothermic peak can be accounted for by variations in the conditions of synthesis, analytical conditions, and volatile content of the MAP compared to previous study. The exothermic reaction at >600 °C was not explored as the readily evolved volatile content is more likely to be affected by Cr sorption.

Thermal analysis of Cr sorbed MAP

The effect of Cr sorption on the temperature of the endothermic transition and total mass loss from the sample was determined by the comparison to that of unreacted MAP (Fig. 4a, b). For MAP precipitated in the presence of aqueous Cr, the temperature of the endothermic peak is mostly within error, with a maximum at 50 μM Cr (Fig. 4a). The total mass loss from the sample is close to that of unreacted MAP, but is consistently higher for initial Cr concentrations >5 μM (Fig. 4b), with a greater effect of Cr(III) compared to Cr(VI).

Fig. 4
figure 4

DSC and TG results for Cr sorbed MAP compared to unreacted MAP. a Temperature of the endothermic transition, b overall mass loss from the sample, c temperature and d mass loss at the TG inflection point

Full size image

The effect of Cr sorption can also be assessed by examining the inflection point in the TG curve. This feature is indicative of the greatest mass loss from the sample. The temperature of the inflection point and the accompanying mass loss increases significantly for Cr-reacted MAP compared to unreacted MAP (Fig. 4c, d). The effect is more pronounced compared to that observed for the endothermic transition and overall sample mass loss (Fig. 4a, b). Results indicate that sorption of both Cr(III) and Cr(VI) shifts the temperature of maximum MAP mass loss to higher temperature. This temperature shift is also accompanied by an overall increase in mass loss from the sample. This effect is more pronounced for Cr(III) relative to Cr(VI) reacted MAP samples, particularly at higher initial Cr concentration. Results confirm that sorption of Cr affects the decomposition process, and thus the stability of MAP.

Volatile content of Cr sorbed MAP

Due to the similarity in molecular mass of H2O(g) and NH3(g), the relative fraction of these constituents in the sample cannot be determined by TG alone. Coupling TG with FT-IR allows the volatile content of the MAP substrate to be calculated. This is accomplished by correlating the relative peak areas of H2O(g) and NH3(g) in the FT-IR spectra with the mass loss from the TG curve at the relevant temperature. For unreacted MAP, the volatile content is calculated as 7.3 mol, close to the stoichiometric value of 7 mol of H2O and NH4 +, and likely within error (Table 1).

Table 1 Moles of evolved H2O(g) and NH3(g) for synthesized MAP compared to theoretical MAP, and for Cr sorbed MAP as determined from combined TG and FT-IR analysis
Full size table

For all Cr sorbed MAP samples the total volatile contents are similar to that of unreacted MAP (Table 1), which is consistent with small variations in observed total mass loss for all samples (Fig. 4b). However, for some samples there are notable changes in the relative moles of H2O(g) and NH3(g) evolved compared to unreacted MAP. For Cr(III) samples with initial concentrations of 10–50 μM, and the 5 μM Cr(VI) sample, the H2O(g) evolved decreases, and that of NH3(g) increases by approximately 1 mol, relative to unreacted MAP. For the 1 μM Cr(VI) sample the decrease in the number of H2O(g) and increase in that of NH3(g) is ~2 mol. These changes in the volatile content of MAP in the presence of Cr may be indicative of substitution of Cr into the mineral structure. If this is the case, results further suggest that this substitution occurs over a limited concentration range for both Cr(III) and Cr(VI).

Mechanisms of Cr substitution

For solids in which no effect of Cr on the bulk MAP composition was observed, Cr sorption to the mineral surface may dominate. At 1–5 μM Cr(III) and 10–100 μM Cr(VI) adsorption may be the primary mechanism, with Cr retained in the near surface region of MAP. For 100 μM Cr(III), precipitation of amorphous Cr solids may be an additional mechanism of Cr removal from solution. For both adsorption and surface precipitation, as Cr is not incorporated into the MAP mineral structure, minimal effects on the MAP composition would be expected.

The 10–50 μM Cr(III) and 1–5 μM Cr(VI) samples, which show changes in volatile content indicate that Cr substitution into the MAP structure may be a dominant mechanism. The MAP structure is comprised of Mg (H2O)6 octahedra linked to NH4 + and PO4 3− tetrahedra by hydrogen bonding [27]. It is anticipated that Cr(III), which forms cationic species in solution will substitute at the Mg(II) site, and that Cr(VI) anions will substitute for P(V). Thus, the volatile content of Cr sorbed MAP can be used to propose the mechanisms of substitution. Based on equilibrium solution chemistry calculations, the dominant aqueous species for Cr(III) and Cr(VI) are Cr(OH) +2 (aq) and CrO4 2−(aq), respectively. Assuming interaction of the aqueous species with the precipitating mineral, and taking into account the addition of NH4 +, and the loss of H2O(s) from the structure from TG and FT-IR, the following substitutions are proposed:

$$ {\text{Cr}}({\text{OH}})_{2}{}^{ + } + {\text{NH}}_{4}{}^{ + } = {\text{Mg}}({\text{H}}_{2} {\text{O}}){}^{{2 + }} $$
(1)
$$ {\text{CrO}}_{4}{}^{2-} + {\text{NH}}_{4}{}^{ + } = {\text{PO}}_{4}{}^{3 - } + {\text{Mg}}\left( {{\text{H}}_{ 2} {\text{O}}} \right){}^{ 2+ } $$
(2)

In (1) the Cr (OH)2 + cation substitutes for Mg2+ in the structure, with the incorporation of additional NH4 + for charge balance. Water of crystallization (H2O(s)) may be lost from the structure to accommodate the Cr species, with the Cr hydroxyl ions (OH−) less susceptible to volatilization compared to structural H2O(s). In (2) CrO4 2− substitutes for PO4 3− accompanied by the addition of NH4 + and removal of Mg2+ for charge compensation. Once again to accommodate the additional ions, H2O(s) is lost from the structure. At 1 μM Cr(VI) a third mechanism may occur:

$$ {\text{HCrO}}_{4}{}^{ - } + 2 {\text{NH}}_{4}{}^{ + } = {\text{PO}}_{4}{}^{3 - } + 2 {\text{Mg}}\left( {{\text{H}}_{ 2} {\text{O}}} \right){}^{ 2+ } $$
(3)

Though HCrO4 −(aq) only represents ~3% of the aqueous species, if crystallizing MAP has a mostly negative surface charge [29], the sorption of this less negative species may be preferable compared to CrO4 2−(aq). In turn, this may proceed when the absolute CrO4 2−(aq) is sufficiently low to limit out competition by this ion, which may be the case at 5 μM Cr(VI). The required charge balance for substitution of the HCrO4 −(aq) species as in (3) is commensurate with the volatile content observed for this sample. It should be noted that the solid Cr concentrations for these samples indicates that only a small fraction of Cr substitutes into the structure (≤4 mg/kg). This suggests that even low concentrations of substituted Cr can propagate significant changes in the MAP the composition.

Implications for MAP use as fertilizer

The use of MAP as a fertilizer has several advantages. The mineral can simultaneously deliver the macronutrients P and N, as well as the secondary nutrient Mg to deficient plants. In addition, MAP recovered from wastewater has proved effective in improving both the yield and P content of plants and crops compared to other P fertilizers [30, 31]. Also, MAP may act as a slow-release fertilizer, reducing leaching and thus the premature loss of nutrients in targeted systems [2, 32].

This research indicates that MAP can be contaminated with Cr during recovery from wastes, with greater association of Cr(III) relative to Cr(VI) with the solid. Relatively low concentrations of substituted Cr can affect the overall composition of MAP. Of concern is the substitution of Cr(VI) for P(V) in the MAP structure. This can be detrimental, as Cr in this oxidation state can displace and follow the same pathways as P in soils and biological systems, with toxic effects for plants and organisms. At all concentrations studied, sorbed Cr was found to affect the thermal properties of MAP. Surprisingly, the mineral appeared to be less susceptible to decomposition. However, at the point of maximum decomposition the mass loss was higher compared to unreacted MAP, suggesting accelerated release of nutrients when decomposition does occur. This can affect its predicted properties as a slow-release fertilizer, as the accelerated release of nutrients can increase the probability of leaching. This cannot only limit nutrient availability to targeted plants, but excessive P and N can act as pollutants in natural ecosystems.

Summary and conclusions

This study outlines a novel application of TG/DSC–FT-IR to study the effect of Cr sorption on the properties of MAP. Results from DSC and TG indicated that sorbed Cr had a minimal effect on both the endothermic transition and total mass loss compared to unreacted MAP. The most significant effect was observed at the inflection point in the TG which shifted to higher temperature and produced greater mass loss compared to unreacted MAP. This indicated that though Cr increased the temperature of maximum decomposition, the loss of volatiles was expedited. The effect of Cr(III) on the thermal properties of MAP was more pronounced than that of Cr(VI), likely due to higher solid reactivity of the former. The volatile content of Cr sorbed MAP was used to provide mechanistic details concerning the Cr sorption mechanism. A significant change in the compositions of selected samples was interpreted as substitution of Cr(III) for Mg(II) or Cr(VI) for P(V) in the MAP structure. For other Cr samples, adsorption or precipitation were likely the dominant mechanisms.

The hyphenated TG/DSC–FT-IR technique elucidated the effect of Cr sorption on the stability of MAP and the overall mineral composition, including the co-sorption and/or loss of species associated with the structure. Of note is that this approach provided mechanistic information at low Cr loadings that would preclude analysis by techniques such as X-ray absorption fine structure spectroscopy (XAFS). The findings predicted potential interactions between Cr and MAP during P recovery from Cr-bearing wastes. Both Cr(III) and Cr(VI) can contaminate MAP even at low initial concentrations, which can prove toxic to plants and organisms. In addition, sorbed Cr can accelerate the release of nutrients from MAP upon decomposition. These findings have implications for the use of MAP fertilizers and subsequent cycling of P and N in geochemical systems.