Efficient Quantification of Soluble and Insoluble Oxalates in Clay Mineral Mixtures

Figures & data

Table 1. Wavenumbers of IR absorption peaks associated with biominerals CaOx monohydrate (COM), CaOx dihydrate (COD) and NaOx.

Wavenumber (cm^−1)	Functional group	Component	Reference
418	metal-O	NaOx	(Parekh et al. 2008)
515	C = O plane bend/Ca-O	Oxalate	(Bhatt and Paul 2008; Kachkoul et al. 2020)
610	metal-O	COD, NaOx	(Lin et al. 2019; Liu et al. 2020; Parekh et al. 2008)
650–670	C = O	COM	(Kachkoul et al. 2020; Lin et al. 2019; Liu et al. 2020; Modlin and Davies 1981)
750–780	C = O out-of-plane bend	COM	(Bhatt and Paul 2008; Chen, Sun, and Zhou 2013; Kachkoul et al. 2020; La Russa et al. 2009; Liu et al. 2020; Modlin and Davies 1981; Rojas-Molina et al. 2015; Sikka et al. 2008)
885	C-C	COM	(Lin et al. 2019; Liu et al. 2020; Modlin and Davies 1981; Parekh et al. 2008)
912		COD	(Lin et al. 2019; Liu et al. 2020)
947–960		COM	(Liu et al. 2020; Modlin and Davies 1981)
1000–1800	C-C	COM	(Lin et al. 2019; Parekh et al. 2008; Schmitt et al. 2018)
1250–1425	C-C	COM	(Parekh et al. 2008; Sikka et al. 2008)
1310–1330	C = O symmetric stretch	Oxalate	(Bhatt and Paul 2008; Chen, Sun, and Zhou 2013; Kachkoul et al. 2020; La Russa et al. 2009; Lin et al. 2019; Liu et al. 2020; Modlin and Davies 1981; Rojas-Molina et al. 2015; Schmitt et al. 2018)
1383	C = O symmetric stretch	Oxalate	(Kachkoul et al. 2020; Sikka et al. 2008)
1590–1620	C = O asymmetric stretch	COM	(Bhatt and Paul 2008; Echigo et al. 2005; Kachkoul et al. 2020; La Russa et al. 2009; Lin et al. 2019; Liu et al. 2020; Modlin and Davies 1981; Parekh et al. 2008; Rojas-Molina et al. 2015; Schmitt et al. 2018; Sikka et al. 2008)
1646	C = O	COD	(Kachkoul et al. 2020; Lin et al. 2019; Liu et al. 2020; Oyebiyi et al. 2018; Schmitt et al. 2018)
1660	C = O asymmetric stretch	Oxalate	(Bhatt and Paul 2008; Kachkoul et al. 2020; La Russa et al. 2009; Schmitt et al. 2018)
3286	O-H stretch (crystal H2O)	COD	(Echigo et al. 2005; Kachkoul et al. 2020)
3400–3500	O-H stretch (crystal H2O)	CaOx	(Bhatt and Paul 2008; Echigo et al. 2005; Kachkoul et al. 2020; Liu et al. 2020; Modlin and Davies 1981; Parekh et al. 2008; Schmitt et al. 2018)

Figure 1. Flow chart showing overview of the development of a mid-infrared (MIR) spectroscopic technique to predict oxalate concentrations in new samples using only spectral information. The model is calibrated by correlation between oxalate concentrations and spectral data of a reference dataset. Several models may be developed due to different possible combinations of predictive model components; thus, the best-performing model is selected by validation procedures.

Table 2. Composition of standard mineral mixtures and oxalic acid solutions.

Mineral (g g^−1)						Solution (M)
CaOx [CaC2O4 · H2O]	NaOx [Na2C2O4]	CaCO3	CaSO4	Kaolinite	SiO2	Oxalic acid
0.000	0.615	0.000	0.000	0.385	0.000	0
0.027	0.580	0.000	0.002	0.370	0.020	0.0003
0.037	0.523	0.044	0.002	0.333	0.061	0.0006
0.051	0.494	0.070	0.006	0.315	0.064	0.0009
0.065	0.455	0.087	0.006	0.295	0.093	0.001
0.084	0.424	0.115	0.006	0.269	0.102	0.0013
0.097	0.395	0.126	0.007	0.252	0.122	0.0016
0.109	0.367	0.145	0.008	0.236	0.134	0.002
0.123	0.347	0.148	0.008	0.213	0.160	0.0023
0.132	0.324	0.160	0.009	0.209	0.166	0.0026
0.142	0.299	0.178	0.008	0.191	0.181	0.003
0.153	0.282	0.182	0.011	0.170	0.202	0.0033
0.161	0.259	0.195	0.012	0.166	0.207	0.0036
0.164	0.231	0.221	0.013	0.153	0.217	0.004
0.180	0.212	0.232	0.014	0.135	0.226	0.0043
0.182	0.200	0.241	0.014	0.127	0.237	0.0046
0.195	0.178	0.246	0.015	0.115	0.252	0.005
0.196	0.168	0.252	0.016	0.109	0.259	0.0053
0.207	0.150	0.262	0.016	0.098	0.268	0.0056
0.218	0.138	0.267	0.016	0.090	0.271	0.006
0.224	0.125	0.277	0.017	0.077	0.280	0.0063
0.226	0.109	0.288	0.017	0.065	0.295	0.0066
0.235	0.095	0.294	0.018	0.060	0.297	0.007
0.236	0.087	0.302	0.019	0.051	0.304	0.0073
0.243	0.071	0.311	0.019	0.045	0.311	0.0076
0.246	0.060	0.314	0.020	0.034	0.325	0.008
0.254	0.041	0.321	0.020	0.031	0.332	0.0083
0.253	0.038	0.329	0.019	0.028	0.332	0.0086
0.259	0.031	0.337	0.019	0.018	0.335	0.009
0.265	0.018	0.345	0.020	0.010	0.342	0.0093
0.270	0.013	0.350	0.021	0.006	0.341	0.0096
0.272	0.000	0.355	0.021	0.000	0.352	0.01
0.531	0.000	0.000	0.000	0.469	0.000
1.000	0.000	0.000	0.000	0.000	0.000
0.000	1.000	0.000	0.000	0.000	0.000
0.000	0.000	0.000	0.000	0.000	1.000
0.000	0.000	0.000	0.000	1.000	0.000
0.000	0.000	0.000	1.000	0.000	0.000
0.000	0.000	1.000	0.000	0.000	0.000
0.208	0.201	0.000	0.000	0.000	0.592

Figure 2. Unprocessed mid-infrared spectra of 0.01 M oxalic acid.

Figure 3. Unprocessed mid-infrared spectra pure solid minerals: calcium oxalate (CaOx, a), sodium oxalate (NaOx, b), kaolinite (c), SiO₂ (d), CaCO₃ (e) and CaSO₄ (f).

Figure 4. Plots for validation of infrared spectral-based models to determine concentrations of (a) calcium oxalate, CaOx and (b) sodium oxalate, NaOx in prepared mineral mixtures. Solid line represents y = x reference line.

Figure 5. Plots for validation of infrared spectral-based models to determine concentrations of oxalic acid in a standard solution. Solid line represents y = x reference line.

Table 3. Figures of merit for models predicting oxalate components in solid and liquid matrices based on MIR spectra of samples.

Component	Units	Matrix	R^2	RMSEP	RMSEPnorm	RPD	RPIQ	Rank	Wavelength regions (cm^−1)	Pre-processing^i	n^ii	Cal: val^iii
CaOx	mmol g^−1	Mineral standard	0.98	0.217	0.02	7.38	5.69	7	7497.9–6781 3209.3–2492.4 1779.7–1062.9	MSC	42	2: 1
NaOx	mmol g^−1	Mineral standard	0.99	0.835	0.04	9.05	13.4	5	6068.3–3922 3209.3–1777.7	SNV	42	2: 1
Oxalic acid	M	H2O standard	0.96	0.001	0.06	2.67	7.47	3	1779.7–1062.9	None	34	1: 2

ⁱSpectral data pre-processing methods include first and second derivative (1^st and 2^nd der.), constant offset elimination (COE), straight line subtraction (SLS), vector normalization (SNV) and multiplicative-scattering correction (MSC).

ⁱⁱn = Total number of samples for calibration and validation, including triplicate values of a single random sample for each of the prepared mineral mixtures and oxalic acid solution standards.

ⁱⁱⁱCal: val = calibration: validation sample splitting. A Kennard-Stone algorithm was used to split calibration and validation sample subsets such that both subsets included similar distributions of the measured component.

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