Comparative Validation of Vanadium(III) Chloride Reduction and Standard Methods for Nitrate Quantification in Organic-Rich Tropical Waters

1. Introduction

Nitrate (NO₃^-) contamination in natural aquatic systems is a major global environmental health challenge and a key proxy for assessing drinking water quality [1,2]. The widespread intensification of agricultural practices, excessive application of nitrogenous fertilizers, untreated domestic wastewater discharges, and inadequate sanitation infrastructure have led to substantial nitrate accumulation in both surface and groundwater resources [3,4]. Ingesting water with elevated nitrate concentrations poses severe toxicological risks to human health. The primary acute risk is infantile methemoglobinemia, wherein gastrointestinal bacterial reduction of nitrate to nitrite (NO₂^-) oxidizes ferrous iron (Fe²⁺) in hemoglobin to ferric iron (Fe³⁺), forming methemoglobin and impairing systemic oxygen transport [5,6]. Furthermore, chronic exposure to low-to-moderate nitrate levels has been epidemiologically linked to endogenous nitrosation, generating carcinogenic N-nitroso compounds associated with colorectal cancer, gastric malignancies, and thyroid dysfunction [7,8]. To safeguard public health, the World Health Organization (WHO) and regulatory bodies such as the US Environmental Protection Agency (US EPA) have established a strict Maximum Permissible Limit (MPL) of 10 mg N-NO₃^-/L (equivalent to 50 mg NO₃^--/L) for safe drinking water [9,10].

In tropical and developing regions across the Global South, drinking water sources are particularly vulnerable to nitrate pollution because of rapid land-use transitions, dense agricultural catchments, and shallow aquifers [11,12]. Recent environmental assessments across tropical ecosystems have documented alarming variations in nitrate levels. For instance, in the karstic Yucatan aquifer in Mexico, groundwater nitrate concentrations used for public supply frequently range from 7 mg/L to over 150 mg NO₃^-/L, with more than 50% of rural drinking wells exceeding international safety thresholds [13,14]. Similarly, studies in agricultural watersheds across Brazil, Sub-Saharan Africa, and Southeast Asia have reported surface and groundwater nitrate levels reaching up to 35-80 mg N-NO₃^-/L [15,16]. In this tropical context, Colombian drinking water sources show dynamic nitrate concentrations ranging from <0.5 mg N-NO₃^-/L in pristine Andean headwaters to >25 mg N-NO₃^-/L in peri-urban and agricultural catchments [17,18]. Crucially, aquatic systems throughout the tropics are naturally enriched with high concentrations of Natural Organic Matter (NOM)—predominantly humic and fulvic acids—derived from rapid biogenic turnover, high soil leaching rates, and dense rainforest runoff [19,20]. This pervasive presence of dissolved organic carbon creates a complex analytical matrix that severely compromises standard water quality monitoring protocols worldwide.

To quantify nitrate in aqueous matrices, analytical chemistry offers a spectrum of methodologies, each governed by distinct trade-offs between matrix resilience, operational cost, and analytical throughput. Direct ultraviolet (UV) spectrophotometry at 220 nm—paired with a secondary baseline correction at 275 nm—remains widely adopted in routine screening due to its rapid execution, low operational cost, and reagent-free nature [21]. However, this direct optical technique is intrinsically vulnerable to spectral overlap from NOM, surfactants, and dissolved transition metals (Fe³⁺, Cr⁶⁺); consequently, standard reference protocols explicitly mandate rejection whenever organic background correction exceeds 10% of the primary absorbance reading [22]. To circumvent these spectral interferences, reference laboratories frequently rely on separation-based techniques such as ion chromatography (IC) or capillary electrophoresis (CE) [23,24]. Ion chromatography delivers exceptional selectivity and sub-microgram limits of quantification (LOQ < 0.005 mg/L) alongside simultaneous multi-anion determination (F^-, Cl^-, NO₂^-, SO₄^2-); yet, its routine deployment in resource-constrained or high-throughput settings is hindered by high capital expenditure, expensive column consumables, strict operator expertise, and rapid column fouling when analysing unrefined NOM-rich waters [23]. Similarly, while CE minimizes sample and reagent consumption, organic matter adsorption onto capillary walls frequently induces peak distortion and migration time instability [24]. On the other hand, electrometric detection via ion-selective electrodes (ISE) offers a portable, cost-effective tool for field assessments, but it suffers from poor sensitivity (LOQ > 0.5 mg/L), severe signal drift, and cross-reactivity with prevalent anions (Cl^-, HCO₃^-) and low-molecular-weight organic acids [25]. Bridging the gap between high chemical selectivity and operational feasibility, indirect reduction-coupled colorimetric assays based on the Griess reaction (₅₄₀ nm) provide an attractive alternative [26,27]. By chemically or enzymatically converting nitrate to nitrite prior to diazotisation and azo dye coupling, these methods shift signal acquisition entirely into the visible spectrum, effectively immunising the measurement against UV-absorbing background interferences [26].

While the Griess-based assay offers clear theoretical advantages, the operational performance of reduction-coupled spectrophotometry depends heavily on the specific reduction pathway utilized. Three primary systems dominate the literature: heterogeneous cadmium (Cd) reduction, homogeneous vanadium(III) chloride (VCl₃) reduction, and biological reduction via eukaryotic enzymatic nitrate reductase (NaR) [28,29,30,31,32,33,34,35,36,37]. Cadmium reduction has long served as the historical regulatory standard (e.g., APHA 4500-NO₃^- E; US EPA Method 353.2) [28,29]. However, cadmium is a carcinogenic heavy metal associated with severe bioaccumulation risks, occupational hazards, complex waste disposal protocols, and tedious column packed-bed maintenance susceptible to over-reduction [30]. To address these toxicity concerns, homogeneous liquid-phase reduction using acidic VCl₃ at moderate temperatures has emerged as a non-carcinogenic alternative that eliminates column clogging and enables high-throughput microplate formats [31,32,33,34]. Parallelly, enzymatic reduction (NaR) using -NADH as an electron donor represents an eco-friendly "green chemistry" pathway operating under benign physiological conditions [35,36,37]. Nevertheless, biological assays are constrained by high enzyme procurement costs, thermal instability during transport or storage, and potential enzymatic inhibition from complex matrix constituents [38].

However, despite these distinct reduction pathways, systematic, side-by-side comparative validations of their operational figures of merit, analytical limits, and resilience to high natural organic matter loads in complex drinking water sources typical of tropical regions remain lacking. To address this major analytical gap, this study aimed to perform a rigorous comparative evaluation and validation of cadmium, vanadium chloride, and enzymatic NaR reduction methodologies coupled with Griess spectrophotometry for accurate nitrate determination in water for human consumption, utilizing tropical water matrices from Colombia as a representative testing framework. We systematically benchmarked key parameters, including limit of quantification, precision, and susceptibility to natural organic matter interference. Our experimental findings demonstrated that both chemical reduction routes (Cd and VCl₃) achieved superior analytical performance, yielding an LOQ of 0.01 mg/L, high precision (2.0%-3.5% RSD for Cd; 1.0%-3.5% RSD for VCl₃), and negligible interference from dissolved organic matter. Conversely, while the Enzymatic (NaR) method achieved an LOQ of 0.05 mg/L with acceptable precision (2.0-5.0% RSD), it suffered significant inhibition under elevated organic loads. Ultimately, this work provides a definitive, globally translatable analytical framework to guide water quality testing laboratories in selecting robust, matrix-resilient nitrate quantification protocols for organic-rich drinking water sources across tropical and developing regions worldwide.

2. Materials and Methods

2.1. Reagents and Analytical Standards

All chemical reagents utilized were of analytical grade or higher purity and used without further purification. Ultra-pure water (Millipore, Bedford, MA, USA) was used for reagent preparation, serial dilutions, and blank controls. A certified commercial primary nitrate standard solution (1000 mg N-NO₃^-/L, Certipur®, Merck, Darmstadt, Germany) was used to construct working calibration curves (0.005-10.0 mg N-NO3-/L). Colorimetric reagents for the Griess reaction were prepared, Sulfanilamide Reagent: 1.0%(w/v) sulfanilamide dissolved in 1.2 M hydrochloric acid (37%); NEDD Reagent: 0.1%(w/v) N-(1-naphthyl)ethylenediamine dihydrochloride dissolved in ultra-pure water and stored in amber glass bottles at 4°C.

For natural organic matter (NOM) interference assessments, commercial Suwannee River Natural Organic Matter (SRNOM 2R101N, International Humic Substances Society) and humic acid sodium salt (Sigma-Aldrich) were used to prepare fortified matrices with Dissolved Organic Carbon (DOC) levels ranging from 1.0 to 20.0 mg C/L.

2.2. Sampling and Characterization of Tropical Water Matrices

To evaluate method resilience in organic-rich matrices, surface water and groundwater samples were collected from drinking water treatment plant intakes across agricultural catchments in Colombia (Risaralda, Cauca Valley, and Andean plateau regions). Sampling adhered to ISO 5667-3 guidelines. Samples were gathered in high-density polyethylene (HDPE) containers, chilled at 4°C, and filtered through 0.45m polyethersulfone membrane filters within 6 h of collection. Matrix characterization included pH and electrical conductivity (HQ40d, Hach, USA), DOC via catalytic combustion oxidation (TOC-L, Shimadzu, Japan), and Specific UV Absorbance at 254 nm SUVA-254 using a UV-Vis spectrophotometer (Genesys 50, Thermo Fisher Scientific, USA).

2.3. Analytical Methodologies and Optimization Protocols

Spectrophotometric absorbance measurements for all reduction pathways were recorded at = 540 nm and 220/275 nm for direct UV screenin using a double-beam UV-Vis spectrophotometer (Genesys 50, Thermo Fisher Scientific, USA) equipped with 1 cm quartz cuvettes. Prior to chemical or enzymatic reduction, all natural water samples were screened for background nitrite (NO₂^-) using the direct Griess reaction without reductants. Pre-existing nitrite concentrations across all tested drinking water sources were consistently below the limit of quantification (<0.010 mg N-NO₂^-/L), confirming that measured chromophore absorbances reflected total nitrate reduction.

2.3.1. Standard Reference Methods (APHA Standard Methods): Benchmarking was performed against three official reference methods codified in Standard Methods for the Examination of Water and Wastewater (APHA/AWWA/WEF):

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Direct UV Spectrophotometry (APHA 4500-NO3^- B): Primary absorbance was measured at 220 nm, with background organic matter interference subtracted via secondary reading at 275 nm.

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Cadmium (Cd) Reduction Method (APHA 4500-NO₃^- E): Samples (25 mL) buffered with ammonium chloride (NH₄Cl-EDTA, pH 8.5) were passed through a glass column packed with copperized cadmium granules (Cd-Cu) at a flow rate of 7.0-10.0 mL/min. The eluate was reacted with sulfanilamide and NEDD prior to absorbance measurement at 540 nm.

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Enzymatic Nitrate Reductase (NaR) Method (APHA 4500-NO₃^- J): Biocatalytic reduction was conducted using eukaryotic Nitrate Reductase (AtNaR2) in 50 mM potassium phosphate buffer (pH 7.2) with 100M -NADH as the electron donor. Samples were incubated at 25°C for 30 min, followed by color development with sulfanilamide and NEDD at 540 nm.

2.3.2. Vanadium(III) Chloride (VCl₃) Method and Parameter Optimization

Homogeneous reduction using acidic VCl₃ coupled with the Griess reaction was systematically evaluated as an alternative pathway. Factorial optimization trials were conducted across three critical reaction parameters to establish maximum reduction kinetics and stoichiometric conversion:

- Acidity (pH / HCl Concentration): Evaluated across hydrochloric acid concentrations ranging from 0.1 M to 3.0 M HCl (pH 0.5-2.0).

- Incubation Temperature: Tested at 20°C (ambient), 37°C, 50°C, 60°C, and 80°C using thermostated water baths.

- Reaction Kinetics / Incubation Time: Evaluated at time intervals of 15, 30, 45, 60, and 90 min.

Under optimized conditions (HCl 1.0 M, 60°C incubation for 45 min), 1.0 mL of water sample was mixed with 1.0 mL of a combined reagent mixture containing acid VCl₃ solution (0.8%w/v prepared as 400 mg VCl₃ in 50mL 1 M HCl), 1.0% sulfanilamide, and 0.1% NEDD (2:1:1 v/v/v). Reactions were cooled to 20°C prior to spectrophotometric reading at 540 nm.

2.4. Method Validation and Precision Assessment

Validation protocols were executed in accordance with IUPAC guidelines and ISO/IEC 17025 standards to compare the performance of the three APHA reference methods (APHA 4500-NO₃^- B, E, and J) against the optimized VCl3 alternative method. Limit of Quantification (LOQ): Verified experimentally using 10 independent matrix blanks spiked at low concentration levels: $\mathit{LOQ}={\displaystyle \raisebox{1ex}{$10\mathrm{*}\mathit{\sigma b}$}\!\left/ \!\raisebox{-1ex}{$S$}\right.}$. where σb is the standard deviation of replicate blank measurements (n = 10) and S is the slope of the calibration line. Precision Multi-Level Evaluation: Precision was systematically assessed across three fortified concentration levels (0.1, 1.0, and 5.0 mg N-NO₃^-/L) in synthetic and real drinking water matrices, with Relative Standard Deviation (%RSD):

-Repeatability (Intra-day Precision): Measured by analyzing n = 6 consecutive replicates under identical experimental conditions (same analyst, instrument, and day).

-Intermediate Precision (Inter-day Precision): Evaluated over 2 consecutive days (n = 5) incorporating daily reagent preparations, independent calibration curves, and variable environmental conditions.

-Reproducibility (Inter-laboratory / Multi-analyst Precision): Assessed across independent analytical runs conducted by three different analysts operating two distinct double-beam spectrophotometric instruments.

Natural Organic Matter (NOM) Interference Evaluation: Matrix resilience against organic background interference was evaluated by fortifying real drinking water samples with humic acids (DOC = 0-20 mg C/L). Percentage recovery (%R) was calculated to quantify matrix suppression or signal enhancement.

2.5. Statistical Analysis

All analytical determinations were performed in triplicate (n = 3), and results are presented as mean ± standard deviation. Differences in analytical performance metrics (LOQ, repeatability, intermediate precision, reproducibility, and NOM interference) between the three APHA standard methods and the optimized VCl₃ method were evaluated using One-Way Analysis of Variance (ANOVA) followed by Tukey’s Honest Significant Difference (HSD) post-hoc test (p < 0.05). Data processing, linear regressions, and statistical modeling were performed using Excel and SPSS v24 software.

3. Results

3.1. Characterization of Tropical Drinking Water Matrices

Physicochemical analysis of surface water and groundwater sources collected across tropical catchments in Colombia confirmed a high background load of Natural Organic Matter (NOM), typical of tropical aquatic ecosystems. Dissolved Organic Carbon (DOC) concentrations ranged from 2.4 mg C/L in pristine upland sources to 18.5 mg C/L in agricultural catchments. Specific UV Absorbance (SUVA 254) values ranged from 2.1 to 4.8 L/(mg m), indicating a high proportion of hydrophobic, aromatic humic and fulvic structures capable of causing severe optical and chemical interferences during nitrate determination. Sample pH remained near neutral (6.4-7.2), with electrical conductivity values between 115 and 480 S/cm.

3.2. Single-Factor and Kinetic Optimization of the Vanadium(III) Chloride Reduction Method

Systematic optimization of the homogeneous liquid-phase VCl₃ reduction pathway was performed using a 1.0 mg N-NO₃^-/L standard solution to establish the precise boundary conditions required for complete stoichiometric reduction of nitrate (NO₃^-) to nitrite (NO₂^-) and subsequent color development via the Griess reaction. The influence of hydrochloric acid concentration (HCl), reaction temperature, and incubation time on color development at  = 540 nm is detailed below and summarized in Table 1.

3.2.1. Effect of Medium Acidity

The single-phase reduction of nitrate by VCl₃ relies on an strongly acidic environment to maintain vanadium in its active +3 oxidation state and promote electron transfer. Varying HCl concentrations from 0.1 M to 3.0 M (pH 1.0-0.5) revealed a pronounced bell-shaped response curve. At low acidity (0.1-0.5 M HCl), incomplete dissolution of the VCl₃ reagent occurred, leading to sluggish reduction kinetics and a maximum reduction yield of only 32.0%-80.0% (absorbance < 0.280 AU). Quantitative stoichiometric conversion (100.0% yield, 0.350±0.005 AU) was achieved within the narrow acidity window of 0.8-1.2 M HCl. Increasing acidity beyond 2.0 M HCl induced a progressive decline in absorbance (85.1% yield at 2.0 M; 75.1% yield at 3.0 M). This signal suppression is attributed to acid-catalyzed protonation and destabilization of the diazo intermediate, which impairs complete coupling with the NEDD reagent and promotes fading of the magenta azo dye. Consequently, 1.0 M HCl was established as the optimal acidity level.

3.2.2. Thermal Profile and Incubation Kinetics

Because the homogeneous reduction of nitrate by VCl₃ exhibits a high activation energy, temperature and time were optimized simultaneously to achieve rapid analysis without compromising azo dye stability. At ambient (20°C) and physiological (37°C) temperatures, the reaction proceeded extremely slowly, yielding only 30.0% and 58.0% conversion after 45 min, respectively. Raising the temperature to 50°C improved kinetics (90.0% conversion), while 60°C provided the thermal energy necessary for complete quantitative conversion (100.0%). However, elevating the reaction temperature to 80°C caused thermal degradation of the azo chromophore, evidenced by a 15.1% drop in net absorbance (0.297±0.014 AU). Time-course trials at 60°C demonstrated a steady increase in absorbance during the first 30 min (89.1% conversion), reaching a stable kinetic plateau at 45 min (100.0%, 0.350 AU). Extending the incubation period up to 90 min resulted in no significant change in signal intensity (p > 0.05), confirming high chromophore stability once formed.

3.3. Analytical parameters of the methods

Calibration curves were constructed across the dynamic range of 0.005-10.0 mg N-NO₃^-/L for all evaluated methodologies. As presented in Table 2, all methods demonstrated acceptable linear determination coefficients (R² ≥ 0.998). However, substantial differences were observed in sensitivity and lower operational limits. Both chemical reduction methods—Cadmium (APHA 4500-NO₃^-E) and the optimized Vanadium (VCl₃) method—achieved the lowest LOQ of 0.010 mg N-NO₃^-/L. The Enzymatic method (APHA 4500-NO₃^-J) yielded a higher LOQ of 0.050 mg N-NO₃^-/L, governed by blank signal variability. Direct UV Spectrophotometry (APHA 4500-NO₃^- B) exhibited an LOQ of 0.100 mg N-NO₃^-/L in clean matrices, but failed to establish a reliable quantification limit in high-NOM samples due to baseline noise.

The precision of the analytical methodologies was systematically evaluated across three fortified nitrate concentrations (0.1, 1.0, and 5.0 mg N-NO₃^-/L) in synthetic and real drinking water matrices. As summarized in Table 2, the optimized homogeneous VCl₃ reduction method demonstrated superior intra-day repeatability (n = 6), yielding relative standard deviation (%RSD) values between 1.0% and 2.2%. This performance was slightly more precise than the standard cadmium column reduction method (APHA 4500-NO₃^- E), which produced repeatability values ranging from 2.0% to 3.1% RSD. The Enzymatic method (APHA 4500-NO₃^-J) exhibited moderate dispersion with repeatability values of 2.1-4.2% RSD, whereas direct UV spectrophotometry (APHA 4500-NO₃^- B) showed the highest variance (3.5-6.8% RSD), primarily caused by optical scattering and short-term light source fluctuations.

Evaluation of intermediate precision over two consecutive testing days confirmed the long-term analytical stability of the homogeneous VCl₃ pathway (%RSD = 1.5-4.5%), matching the performance of cadmium reduction (2.5-3.5% RSD) as presented in Table 1. The enzymatic method displayed greater inter-day variance (3.2%-5.0 %RSD), which was attributed to subtle day-to-day variations in enzyme activity and reagent solution stability. Crucially, multi-analyst and inter-laboratory reproducibility assessments revealed significant operational differences between the chemical reduction techniques. While cadmium column reduction experienced a drop in reproducibility (4.5%-6.2% RSD) due to operator-dependent variables such as column packing density and manual flow rate regulation, the homogeneous liquid-phase VCl₃ method maintained excellent reproducibility (2.2%-3.8 %RSD, p = 0.42). The absence of column physical constraints in the VCl₃ protocol ensures robust, operator-independent performance across different laboratory environments.

The selectivity and robustness of each methodology against matrix interference was evaluated by spiking real drinking water samples with incremental concentrations of humic substances, establishing a dissolved organic carbon (DOC) gradient from 0 to 20.0 mg C/L. As detailed in Table 2, direct UV spectrophotometry exhibited poor selectivity when applied to organic-rich matrices. At DOC concentrations exceeding 3.0 mg C/L, background absorbance at 275 nm routinely surpassed 10% of the primary reading at 220 nm, violating the quality control threshold mandated by regulatory standards. This spectral overlap produced severe signal overestimation, yielding erroneous recoveries between 135% and 185% or negative values upon over-correction.

A distinct form of matrix interference was observed with the enzymatic reduction assay (APHA 4500-NO₃^-J). Although the biocatalytic method performed with adequate selectivity under low-organic conditions (DOC < 2.0 mg C/L), elevated organic carbon levels severely compromised method robustness through signal suppression (p < 0.01). As recorded in Table 2, mean spike recovery for the enzymatic assay dropped to 72.4 ±3.1% at DOC = 10.0 mg C/L and further declined to 54.1±4.2% at DOC = 20.0 mg C/L. This severe inhibition indicates that complex humic and fulvic macromolecules physically bind or deactivate the active site of the eukaryotic nitrate reductase (NaR) enzyme, restricting electron transfer from -NADH to the nitrate substrate.

In sharp contrast to optical and biological approaches, both chemical reduction pathways demonstrated exceptional matrix tolerance and high analytical selectivity. As shown in Table 2, cadmium column reduction and the optimized VCl₃ method maintained quantitative spike recoveries across all organic loads up to 20.0 mg C/L, achieving mean recovery values of 98.5±2.2% and 101.2 ±1.8%, respectively (p > 0.05). By stoichiometrically converting nitrate to nitrite and shifting the optical reading into the visible spectrum ( = 540 nm) via azo dye formation, both chemical systems completely bypass UV-absorbing organic background interferences. Consequently, the optimized VCl₃ method combines the high selectivity and matrix tolerance of Cadmium reduction with lower reagent toxicity and superior reproducibility, establishing it as the most robust alternative for organic-rich tropical drinking water sources.

4. Discussion

Accurate quantification of nitrate (NO₃^-) in tropical aquatic systems is routinely compromised by high background concentrations of natural organic matter (NOM), creating an analytical challenge for drinking water compliance and environmental monitoring. The experimental findings of this study provide a comprehensive comparative evaluation of regulatory standard methods (APHA 4500 NO₃^- B, E, and J) against an optimized homogeneous vanadium(III) chloride (VCl₃) reduction assay. By systematically benchmarking operational parameters, kinetic behaviors, figures of merit, and matrix tolerance across real and synthetic tropical water matrices, this discussion elucidates the underlying chemical mechanisms governing method performance, highlighting the trade-offs between analytical selectivity, operational complexity, and environmental sustainability.

The single-step quantitative conversion of nitrate (NO₃^-) to nitrite (NO₂^-) by trivalent vanadium (V³⁺⁾ represents a multi-electron redox coupling reaction governed by both thermodynamic driving forces and kinetic barriers. In an acidic aqueous medium, V³⁺ acts as a two-electron donor, undergoing oxidation to oxovanadium species (VO²⁺ or VO₂⁺) while reducing nitrate according to the net stoichiometry:

Our optimization findings confirm that this single-phase reaction requires precise temperature and acidity control. The requirement for elevated incubation temperatures (60°C) stems from the high kinetic activation energy (E_a) associated with cleavage of the nitrogen-oxygen bond in the planar nitrate anion. At temperatures below 50°C, electron transfer from V³⁺ proceeds too slowly for routine high-throughput workflows, explaining the sub-quantitative yields (<60%) observed at ambient and physiological temperatures. Conversely, heating beyond 60°C introduces thermal instability. The observed 15.1% drop in absorbance at 80°C is consistent with the thermal decomposition of the diazonium intermediate and the irreversible degradation of the resulting magenta azo dye ( = 540 nm), a phenomenon previously reported in high-temperature colorimetric optimization studies [31,33].

Acidity plays a dual role in this analytical system. First, maintaining an acid concentration of 1.0 M HCl (pH ≈0.8) prevents the hydrolysis and precipitation of V³⁺ as insoluble vanadium hydroxides, maintaining high redox reactivity [32]. Second, the subsequent diazotization of sulfanilamide requires a strongly acidic environment to generate the reactive electrophilic diazonium cation. However, acidity exceeding 2.0 M HCl suppressed color development. Excess hydrogen ion concentration protonates the amine group of the coupling reagent (N-(1-naphthyl)ethylenediamine dihydrochloride, NEDD), decreasing its nucleophilicity and slowing the electrophilic aromatic substitution required to form the final chromophore [26]. Thus, maintaining 1.0 M HCl at 60 °C for 45 min achieves an optimal balance between rapid reduction kinetics and chromophore stability.

A critical insight from our validation assays is the structural advantage of homogeneous liquid-phase reduction over traditional heterogeneous column methods. While both cadmium (APHA 4500-NO₃^- E) and VCl₃ assays achieved an identical limit of quantification (LOQ = 0.010 mg N-NO₃^-/L), the VCl₃ pathway demonstrated superior multi-analyst reproducibility (2.2%-3.8% RSD) compared to cadmium column reduction (4.5%-6.2% RSD). This disparity in precision stems directly from the fluid dynamic and surface chemistry constraints inherent to packed-bed column reactors. Cadmium reduction relies on copperized cadmium granules packed inside glass columns, where subtle variations in packing density, fluid channeling, micro-bubble entrapment, and manual flow-rate control (7.0-10.0 mL/min) introduce significant operator-dependent variance. Furthermore, active cadmium sites undergo progressive surface oxidation and passivation over time, requiring periodic column stripping and reactivation with copper sulfate (CuSO₄), while sub-optimal contact times can induce over-reduction of nitrate past nitrite to ammonium (NH₄⁺), introducing negative analytical bias. In contrast, the VCl₃ methodology operates entirely as a single-phase liquid assay in solution. By eliminating column packing, flow-rate dependency, and surface passivation, the VCl₃ protocol delivers a good repeatability (%RSD < 2.2%) and seamless transferability across different analysts and spectrophotometric platforms. Moreover, this homogeneous liquid-phase format makes the VCl₃ assay highly amenable to miniaturization into 96-well microplate formats, unlocking automated, high-throughput analysis that is physically unachievable with manual cadmium columns [34].

Direct UV measurement at 220 nm relies on the electronic transition (π→π*) of the nitrate ion. However, tropical NOM contains abundant aromatic rings, conjugated double bonds, and phenolic carboxyl groups that absorb strongly across the 200-300 nm spectrum [19,21]. Although standard protocols apply a secondary correction reading at 275 nm, this linear mathematical subtraction assumes a constant optical slope for organic matter. In water matrices with SUVA₂₅₄ > 3.0 L/mg m, complex humic structures display non-linear UV absorption profiles [22]. When organic background correction exceeds 10% of the primary reading, the algorithm breaks down completely, yielding gross overestimations (>150% recovery) or unphysical negative values. This confirms that direct UV spectrophotometry is fundamentally unsuited for routine testing in organic-rich tropical surface waters.

While the Enzymatic Nitrate Reductase (NaR) method shifts detection to the visible spectrum (540 nm), it suffered severe signal suppression under high organic loads, with recoveries dropping to 54.1±4.2% at DOC = 20.0 mg C/L. This pronounced inhibition arises from complex biochemical interactions between humic macromolecules and eukaryotic protein structures. High-molecular-weight humic and fulvic acids possess extensive hydrophobic domains and polyanionic carboxylate groups that non-specifically adsorb onto hydrophobic surface patches of the NaR enzyme, sterically blocking substrate access to the molybdenum-pterin active site [35,38]. Concurrently, these polyphenolic humic substances act as natural protein precipitants and chelators, inducing partial conformational changes and tertiary folding alterations that diminish catalytic activity [36]. Furthermore, dissolved organic matter can intercept or non-specifically oxidize the reduced -nicotinamide adenine dinucleotide (-NADH) co-factor, depleting the essential electron donor required to drive the biocatalytic reduction cycle [37]. Together, these synergistic mechanisms severely compromise the selectivity and quantitative accuracy of enzymatic assays in organic-rich waters.

In contrast to optical and biocatalytic methods, both chemical reduction pathways (Cd and VCl₃) demonstrated complete immunity to organic matrix interferences (98.5%-101.2% recovery up to 20.0 mg C/L). Chemical reduction converts NO₃^- to NO₂^-, which is subsequently coupled to form an intensely colored azo dye with a sharp absorption maximum at = 540 nm. Because dissolved organic matter exhibits negligible background absorbance in the visible spectrum beyond 500 nm, shifting signal acquisition to 540 nm provides complete spectral selectivity [26,32]. Furthermore, the strongly acidic conditions (1.0 M HCl) and chemical reductants utilized in the VCl₃ assay are unaffected by polyphenolic binding or matrix-induced protein denaturation, ensuring high analytical robustness across diverse natural water matrices.

The selection of routine nitrate monitoring protocols involves balancing analytical performance against occupational safety, environmental impact, and cost considerations, particularly in resource-constrained regions of the global south. Cadmium reduction (APHA 4500-NO₃^- E) has long been mandated as the benchmark standard by regulatory agencies worldwide. However, Cadmium is a highly toxic, bioaccumulative heavy metal classified as a group 1 human carcinogen under international regulations [30]. The routine operation of cadmium columns generates significant volumes of hazardous chemical waste containing toxic Cd²⁺ and Cu²⁺ ions, requiring complex, costly waste disposal infrastructure that is often lacking in decentralized testing laboratories in developing countries. Although the enzymatic assay (APHA 4500-NO₃^- J) represents an eco-friendly "green chemistry" alternative that eliminates toxic metal waste [35], its high reagent cost (primarily driven by commercial NaR enzyme and -NADH co-factors), requirement for continuous cold-chain storage (<-20°C), and susceptibility to NOM inhibition limit its applicability for large-scale, routine monitoring in tropical rural catchments.The optimized Vanadium(III) chloride (VCl₃) methodology emerges as the most balanced, practical solution. While Vanadium compounds require standard chemical handling, VCl₃ presents a significantly lower toxicological and bioaccumulative footprint than Cadmium [31,34]. Its liquid-phase homogeneous format eliminates column generation, reduces reagent consumption, utilizes low-cost inorganic salts, and maintains long-term solution stability at room temperature. Combined with its LOQ of 0.010 mg N-NO₃^-/L, excellent reproducibility (2.2-3.8% RSD), and complete immunity to NOM interference, the optimized VCl₃ method provides a robust, high-throughput, and globally translatable analytical protocol. Adopting this method in routine water quality testing laboratories can significantly enhance monitoring accuracy for drinking water systems in Colombia and other organic-rich tropical catchments worldwide.

While this study provides a rigorous validation of nitrate quantification pathways in organic-rich matrices, several analytical limitations should be acknowledged. First, although the evaluated surface and groundwater samples covered a broad range of dissolved organic carbon (DOC= 2.4-18.5 mg C/L) and aromaticities (SUVA₂₅₄ = 2.1-4.8 L/mg-m) representative of tropical catchments, testing was focused on specific Colombian hydrological basins; variations in specific organic matter fractions, such as highly hydrophilic dissolved organic nitrogen or microbial exopolymeric substances from non-tropical regions, were not exhaustively evaluated. Second, the optimized VCl₃ homogeneous reduction assay requires a thermostated incubation step (60°C for 45 min), which introduces a minor thermal energy requirement and operational delay compared to instantaneous optical screening methods, although this is easily accommodated using standard water baths or heated microplate readers. Third, matrix interference testing focused primarily on natural organic matter and humic substances; potential cross-interferences from extreme concentrations of specific transition metals (e.g., Fe³⁺ or Mn²⁺ exceeding 50 mg/L) or extreme saline gradients were not systematically mapped across all matrix types, although complexation with EDTA in the Griess reagent mixture is routinely effective at masking transition metal interferences [28,33]. Finally, the enzymatic reduction protocol was evaluated using a single commercial eukaryotic Nitrate Reductase variant (AtNaR2); alternative recombinant enzymes or prokaryotic nitrate reductases engineered for higher thermal and chemical stability were not investigated and may exhibit different matrix tolerance profiles.

5. Conclusions

This study presents a comprehensive comparative validation of regulatory standard methodologies (APHA 4500-NO₃^- B, E, and I) and an optimized homogeneous Vanadium(III) chloride (VCl₃) reduction assay for quantifying nitrate in drinking water sources from organic-rich tropical catchments. By utilizing Colombian water matrices as a representative testing framework for the global south, the research demonstrates that the analytical reliability of nitrate testing depends heavily on the chosen reduction mechanism and its resilience against background natural organic matter (NOM).Experimental validation revealed fundamental operational trade-offs among the evaluated techniques. Direct UV Spectrophotometry (APHA 4500-NO₃^- B proved fundamentally unsuited for tropical drinking water monitoring, failing completely in matrices with dissolved organic carbon (DOC) levels above 3.0 mg C/L due to non-linear background UV absorption. Similarly, while the enzymatic nitrate reductase method (APHA 4500-NO₃^- J) offers an eco-friendly "green chemistry" alternative, it exhibited severe matrix inhibition under elevated organic loads, suffering a recovery drop to 54.1±4.2% at DOC = 20.0 mg C/L due to humic-induced protein binding and co-factor oxidation. Conversely, chemical reduction pathways (Cd and VCl₃) demonstrated complete immunity to organic matrix interferences, maintaining quantitative spike recoveries (98.5%-101.2%) by shifting optical signal acquisition entirely into the visible spectrum (= 540 nm).Among the evaluated chemical methods, the optimized VCl₃ homogeneous reduction assay achieved superior overall analytical performance, yielding an limit of quantification (LOQ of 0.010 mg N-NO₃^- / L, excellent intra-day repeatability (1.0-2.2% RSD), and superior multi-analyst reproducibility (2.2-3.8% RSD) compared to cadmium column reduction (4.5-6.2% RSD). By operating in a single-phase liquid format, the VCl₃ protocol eliminates packed-bed column maintenance, flow-rate dependency, and toxic cadmium waste, while offering a low-cost, high-throughput alternative suitable for microplate automation. Ultimately, this work provides a robust, evidence-based analytical framework to guide water quality testing laboratories in implementing matrix-resilient, globally translatable nitrate monitoring protocols across tropical and developing regions worldwide.