Activation of cadmium under simulated solar illumination and its impact on the mobility of Cd in flooded soils

In waterlogged paddy soils, cadmium (Cd) can be precipitated as cadmium sulfide (CdS) under reductive environment, thereby limiting the absorption of Cd by plants. Multiple environmental factors (such as water, pH, and Eh) played a role in the control of Cd mobility and bioavailability. In this study, we investigated the influence of the solar irradiation on the photodissolution of synthetic CdS-montmorillonite composites (CdS-M) in solution and the stability of Cd in natural soil. The release kinetic of Cd2+ showed that after the irradiation of simulated sunlight, CdS-M composites became less stable compared to the dark control. The solar irradiation seemed to enhance the release of Cd2+ from CdS significantly and continuously. Electron paramagnetic resonance (EPR) and quenching experiments confirmed that the photogenerated holes, •O2− and •OH, were possibly involved in the photo-induced release of Cd2+, while the holes was primarily responsible for the reaction. Irradiation under alkaline solution or the presence of DOM, PO43−, CO32−, and urea markedly inhibited the photodissolution process of CdS. The photo-mediated activation of Cd was further confirmed in paddy soil under natural sunlight, with a nearly threefold increase in concentration of extractable Cd during the 15 days of irradiation. This study highlights the importance of photochemical transformation of Cd in the environmental water and soil.


Introduction
Cadmium (Cd) is a non-essential biological element with high-potential toxicity, mobility, and bioaccumulation. Accumulation of Cd in farmland not only decreases the productivities and qualities of crops, but it also poses a threat to ecosystems and human health (Yang et al. 2022). Chronic exposure to low Cd concentrations via intake of Cd-polluted air, water, and foodstuffs has resulted in numerous cases of ill health in humans (Bingham et al. 1975). Anthropogenic activities, such as mineral exploitation, sewage irrigation, and fertilizer application, have greatly increased Cd levels in the water and soil environment (Saini and Dhania 2020). Among all the surveyed contaminants, Cd ranks first in the percentage of soil samples (7.0%) exceeding the soil quality standards in China (Mahar et al. 2016). It is also one of the most harmful and widespread metals in agricultural soil of South China, which is readily absorbed by crop plants and threatens human health through the food chain (Wang et al. 2021).
When Cd enters the soil, it is first adsorbed on soil particles' surface and then can be changed into other Cd forms through various physicochemical or biological processes. In fact, the different forms of Cd often change with environmental conditions, such as soil pH, Eh, dissolved organic matter (DOM), CEC, and organic matter content, thus influencing its fate and bioavailability (Lu 2000;Niu et al. 2013). At present, there are several methods to classify the occurrence forms of heavy metals in soil, and the most widely used and representative morphological analysis method is sequential extraction procedure (BCR) proposed by the European Community Bureau of Reference (Ure et al. 1993), which divides the element in soil or sediment into four fractions: the acid-exchangeable fraction (F1), reducible fraction (F2), oxidizable fraction (F3), and residual fraction (F4). Among them, cadmium associated with organic matter or sulfide (CdS) belong to the oxidizable fraction (F3).
Rice is one of the most important cereal crops in the world and the main food crop in Asia (Carrijo et al. 2017). Because the paddy is continuously flooded or irrigated until harvesting, the soil is usually under anaerobic and reductive conditions. S 2− produced in soil can deposit soluble Cd 2+ in the form of CdS or transform the refractory Cd(OH) 2 and CdO into more refractory sulfide, decreasing the mobility and bioavailability of Cd.
CdS is also a kind of II-IV semiconductor material with direct band gap (2.4 eV) and high absorption, which is highly photoactive under sunlight Swafford et al. 2006). Upon illumination, photoelectrons are excited from their valence band to the conduction band of CdS, forming photoelectron-hole pairs and triggering a series of redox reaction. In previous studies, photocatalysis driven by CdS has received considerable attention as it can be used for environmental remediation and solar energy conversion (Yu et al. 2015). For example, effective photocatalytic degradation of methylparaben (MeP) was obtained in the study of Mohan et al. (2021) using reduced graphene oxide/ cadmium sulfide (RGOCdS) nanocomposite. Furthermore, photocorrosion of CdS nanoparticles or quantum dots and subsequent release of Cd 2+ ions have also been reported Mohan et al. 2021). When the valence band edge was higher than the standard potential for anodic dissolution of cadmium sulfoselenide, the corrosion of CdS took place attributing to the reactions between photogenerated holes and the pigment lattices. The Cd 2+ release from the pigment was further confirmed in river water under natural sunlight, with 38.6 ± 0.1% of the cadmium released within 4 h . Zhang et al. (2020) also found that the S 2− in CdS is vulnerable to the attack of the photogenerated holes, leading to the instability of CdS and release of Cd 2+ . On the basis of these studies, a mechanism can be proposed for the sunlight-induced dissolution of CdS: Except for the photogenerated holes, other active species especially •OH radical can also oxidize CdS, which resulted in the Cd 2+ release (Tang et al. 2014). Such information reveals the presence of a photo-electrochemical system involved in the migration and transformation of Cd in paddy soils, which may increase the release risk of organic binding state Cd.
In conclusion, the photocorrosion of CdS results in the release of Cd 2+ . Given the presence of CdS in paddy soils, the main objective of this study is to explore whether the photocorrosion of CdS in paddy soil under sunlight irradiation will aggravate soil cadmium pollution. In order to simulate CdS in natural soil, the montmorillonite was applied as a carrier to load the prepared CdS particles (CdS-M).The photo-mediated release of Cd 2+ and SO 4 2− from CdS was examined under simulated sunlight. The effect of factors including pH, DOM, phosphate, carbonate, urea, ferrous, and ferric ion on the photodissolution dynamics were further explored to inform the natural release process. Electron paramagnetic resonance (EPR) and quenching experiments were carried out to infer the possible mechanism of photodissolution. In addition, the photo-mediated transformation of CdS in paddy samples was investigated under natural sunlight to support our findings.

Preparation of MMT-loaded CdS
The CdS-M composites were synthesized through the following steps: (i) Na-MMT was prepared by treating 9 g of MMT with 1 M NaCl solution under mechanical stirring for 24 h. Then, the mixed suspension was centrifuged to remove Na-MMT and redispersed to 1 M NaCl. This procedure was repeated 5 times in order to obtain maximum saturation. Then, it was separated from the solution and washed several times with distilled water. (ii) The Na-MMT was mixed with 100 mL of 0.2 M CdCl 2 solution for 2 h at 60 °C in a magnetic mixer. The mixture was then separated from the suspension through centrifugation and rinsed with deionized water several times to remove the excess Cd 2+ . (iii) The Cd 2+ -MMT was redispersed in 100 mL of 0.2 mol/L Na 2 S solution, and the mixture was stirred at 25 °C for 2 h. Clay particles in the solution immediately turned yellow. After that, the obtained CdS-M composites were centrifuged and rinsed three times with absolute ethanol and then dried in 50 °C oven under vacuum overnight. Dried samples were stored in an anaerobic chamber filled with pure N 2 gas prior for use.

Simulated illumination experiments
Solar irradiation was simulated by a 50 W xenon lamp (CELHXF300, AULTT, China). The CdS-M stock suspension was prepared by dispersing 0.02 g CdS-M powder into 100 mL deionized water by ultrasonication for 2 min, and then the resulting solution was placed in a water-circulating jacket beaker at 20 ± 1 °C and irradiated by the Xe lamp from the top without stir. The unadjusted pH of the suspension was 6.0 ± 0.2. At a regular interval, 5 mL sample was withdrawn from the beaker and filtered through a 0.22-μm pore size membrane filter to remove particles for further analysis. To guarantee precise data, all of the experiments were performed at least threefold, and average values were recorded. Dark control experiments were carried out in the same experimental setting with the Xe lamp off and the beaker wrapped with aluminum foil.
The effect of pH on Cd 2+ photo-mediated release was studied in the pH range of 3.98-8.11, with solutions adjusted using 0.1 M HCl and NaOH. To examine the role of different reagents in the reactions, DOM, PO 4 3− , CO 3 2− , and urea were added in the simulated illumination experiments.

Reactive oxygen species detection
The generation of ROS by CdS under simulated solar irradiation was determined by electron paramagnetic resonance method (EPR, A300-10/12, Bruker, Germany) and quenching experiment. The EPR were tested at room temperature with a microwave power of 20 mW, resonance frequency of 9.77 GHz, center field of 3480 G, sweep width of 200 G, modulation frequency of 100 kHz, and modulation width of 1 G. The 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) was used as a spin-trapping agent for •OH and •O 2 − , and the dimethyl sulfoxide (DMSO, 65%) was used as the •OH scavengers during the detection of •O 2 − . The ROS involved in the photoreactions were examined via the quenching experiments using Na 2 S 2 O 3 as the hole scavenger, MV 2+ as the electron scavenger, DMSO as the hydroxyl radical scavenger, and benzoquinone (BQ) as the peroxy radical scavenger.

Soil illumination experiments
To explore the effect of sunlight on soil Cd forms, the photomediated transformation of CdS in natural soil under sunlight was also examined. The soil samples were collected from a paddy field in Changsha, China (113.10°E, 28.38°N) by five-spot sampling method. The depth of the topsoil was 0-20 cm. Soil samples were air-dried, ground, and sieved through a 2-mm nylon sieve. Fifty grams of soil sample were completely mixed with 0.2 g CdS-M composite, which was then added in a serum bottle and submerged by water (1:3 w/v). The serum bottles were sealed by microporous membrane filter and placed outdoors, while the bottles for dark control experiment were wrapped with aluminum foil and protected from light during the experiments. Three replicates were performed for each group. At the interval of every 3 days, the water and soil samples were taken out at 7 p.m. for analysis. Extractable Cd in soil was determined by diethylenetriaminepentaacetic acid (DTPA) extraction by shaking for 16 h at 25 °C. BCR sequential chemical extraction was used to determine the different fractions of Cd in soil.

Analytical method
The Cd 2+ concentration was detected by Optima 8300 ICP-OES (PerkinElmer, USA). The SO 4 2− concentration was determined by ion chromatography (ICS-1000, Dionex, USA) with a Dionex IonPac AS11-HC analytical column (250 × 4 mm). Reagent blanks and sample replicates were used to ensure analytical quality.
The phase composition and crystallinity of CdS-M composite were examined by X-ray diffractometer (XRD, Bruker D8-advance, Germany) with Cu-Kα radiation (λ = 1.5418 Å). The particle morphology was observed on a transmission electron microscopy (TEM, JEM-1230, JEOL, Japan). The surface chemistry of the CdS-M sample was examined using the X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI, Thermo Scientific, USA) equipped with an Al Kα X-ray radiation at 150 W. The element composition of the CdS-M was determined using a FE-SEM (FEI, QUANTA250, USA) equipped with an energy-dispersive X-ray spectroscope (EDX).

Characterization of CdS-M composites
To confirm the successful incorporation of CdS in MMT, the crystalline structure of the MMT and CdS-M composites were characterized by XRD (Fig. 1). The main peak in the diffraction pattern of MMT (2θ = 5.9°) can be attributed to the regular stacking of the silicate layers along the [001] direction, which is normal for montmorillonites (Krauskopf 1967 (Chen et al. 2020). Furthermore, compared to the natural MMT, that peak of the CdS-M was less obvious, and some peaks have disappeared, which indicated that the ordered layered structure of CdS-M was disrupted during loading.
The morphology and particle distribution of CdS on clay were analyzed by HR-TEM. Figure 2a and b provide the images of CdS-M samples containing some darker spots, the putative CdS particles with a few nanometers in diameter. Considering that the cation exchange only occurred in the MMT layers, CdS oligomers are mostly bound in the interlayers of clays, which were not distinguishable. The EDX elemental mapping of the CdS-M composites confirmed the presence of CdS on the MMT support (Fig. 2c). As shown in Fig. 2d, the HR-TEM image showed crystal spacings of 0.34 and 0.36 nm on the CdS-M composites, which were ascribed to the [002] and [100] crystal planes of the hexagonal CdS, respectively. Due to the distinct lattice fringes, CdS particles are easily differentiated from the clay background. EDX element analysis demonstrates that Si, Al, Na, and O from the MMT are presented (Fig. 2e), and distinct peaks of Cd and S from the CdS are also observed. Quantitative analysis results revealed that the CdS-M composite contains Cd (12.92 wt%) and S (7.00 wt%).
The XPS was utilized to elucidate the chemical structure and composition of the CdS-M sample. In addition to the high response of Al and Si in montmorillonite, Cd 3d and S 2p signals were detected in the XPS spectra (Fig. 3a). The deconvoluted high-resolution spectra showed double peaks at 411.4 eV (Cd 3d 3/2 ) and 404.7 eV (Cd 3d 5/2 ), which were considered the characteristic peaks of the Cd 2+ species, whereas the peaks at 168.6 eV (S 2p 1/2 ) and 161.1 (S 2p 3/2 ) were assigned to the spin-orbit double of S 2− (Fig. 3b and c) (Huang et al. 2018). As a result, the characterization by XRD, TEM, EDX, and XPS verified that the CdS was loaded into the MMT layers successfully with the procedures.

Release kinetics of Cd 2+ from CdS-M composite
The release of Cd 2+ and SO 4 2− from the CdS-M composite was examined under both dark and simulated solar irradiation ( Fig. 4a and b). There are small amount of Cd 2+ and SO 4 2− release from the CdS-M suspension in the dark condition within 240 min, which can be ascribed to the ions release during the dispersion process by sonication. This indicated an insignificant oxidative dissolution of CdS in the dark condition. In contrast, the presence of simulated solar irradiation was found to lead to significant and continuous enhance in the release of Cd 2+ and SO 4 2− in the solution. The concentration of Cd 2+ increased to 7.60 ± 0.38 mg/L within a 100-min irradiation and to 9.11 ± 0.23 mg/L after 240 min of irradiation; meanwhile, the concentration of SO 4 2− reached 20.09 ± 0.30 mg/L at the end of the test. The release kinetics of Cd 2+ can be well described by the pseudo-second-order equation. The pseudo-second-rate constants (k, L/mmol·h) was determined by fitting data to the following kinetic equation: The calculated k of Cd 2+ was 0.126 L/mmol·h (R 2 = 0.97), which was 6.3 times higher than that under dark condition (k = 0.020 L/mmol·h, R 2 = 0.90). These results revealed that sunlight played a key role in promoting the release of Cd 2+ from CdS.

Mechanisms for the photodissolution of CdS
The EPR and radicals quenching experiments were carried out to detect the photo transients (such as •OH and •O 2 − ) generated by the CdS under simulated solar irradiation. As illustrated in Fig. 5a, a significant four-line signal with a characteristic intensity of 1:2:2:1 was observed in biochar in the presence of DMPO, thus confirming the generation of •OH (Makino et al. 1991). In the presence of 65% DMSO as the •OH quencher, a six-line EPR signal with an intensity ratio of 1:1:1:1:1:1 was observed, which is the hallmark of the DMPO•OO(H) adduct, indicative of •O 2 − generation (Fig. 5b) (Fang et al. 2017). Additionally, there were no EPR peaks in the same sample without simulated solar irradiation.
As previously reported by Gu et al. (2015), the band gap energy (E g ) of CdS was 2.4 eV, and its conduction and valence bands (CB and VB) edge potentials are located at about − 0.505 and + 1.895 eV, respectively. Therefore, CdS could be excited by the visible-light irradiation and produce the photogenerated electron-hole pairs (Eq. (1)). The CB potential of CdS is more negative than the redox potential •OH/OH − = + 1.99 eV, respectively) (Luo et al. 2017). In consequence, the formation of •OH was most likely generated through the H 2 O 2 reduction pathway (Eqs. (5) and (6)). (1) To determine the contribution of holes, electrons, •OH, and •O 2 − , scavengers including Na 2 S 2 O 3 , MV 2+ , DMSO, and BQ were added into the CdS-MMT photoreaction system, respectively. As shown in Fig. 5c, the photodissolution of CdS under simulated solar was all inhibited after the addition of Na 2 S 2 O 3 , DMSO, and BQ, which decreased from 9.11 to 3.04, 7.38, and 5.54 mg/L, respectively, after 240 min. However, the presence of MV 2+ exerted an obviously promoting effect on the photodissolution of Cd 2+ . MV 2+ can react with Fig. 2 a- photogenerated electrons, reducing the recombination probability of photogenerated electron-hole pairs and consequently leaving more holes involved in the photocatalysis system. This is consistent with the observations in the previous studies Matsumoto et al. 1996). Furthermore, the photodissolution of CdS was also significantly inhibited under the anaerobic condition. Deficient oxygen cannot combine with electrons to yield •O 2 − , leading to more electron recombined with the holes and the inhibited release of Cd 2+ accordingly. The above results showed that the photogenerated reactive species (h + , •O 2 − , •OH, etc.) were possibly involved in the photo-induced release of Cd 2+ (Ge et al. 2015;Hu et al. 2018;Liu et al. 2017Liu et al. , 2020Zhang et al. 2018), while the reaction was predominated by the holes. Based on these, the possible mechanism for the solar-mediated Cd 2+ release from CdS is as follows (Eqs. (7)-(9)):

Influence of environmental factors on the photodissolution of CdS
To further understand the environmentally realistic release process, the effects of environmental factors including pH, DOM, PO 4 3− , CO 3 2− , and urea on the photodissolution of CdS were examined.

pH
As shown in Fig. 6a, the solution pH has an important effect on the photolysis of CdS. The photodissolution rate increased with enhancing the initial pH in acidic condition (from 3.98 to 6.00), which was consistent with the results of Liu et al. (2017). The decreased photodissolution of the CdS at low pH was mainly due to the fact that the reactions produce protons (Eqs. (7)-(9)).
However, the photodissolution drastically decreased as the initial pH increased to 8.11. This could be ascribed to the decreased VB edge of CdS as the pH increased from 6.00 to 8.11, resulting in the reduced photooxidation of CdS and inhibited photo-induced release of Cd 2+ in the solution. Another potential inhibition mechanism is the formation of Cd(OH) 2 precipitates (K sp , Cd (OH) 2 = 7.2 × 10 −15 ) and adsorption of OH − on the CdS-M surfaces under alkaline condition (Lide 2004;Torimoto et al. 1998). These processes could effectively blocked the surface sites suitable for trapping holes, resulting in higher recombination of Fig. 3 a XPS spectrum of montmorillonite loaded with cadmium sulfide; b XPS spectrum of S2p scan; c XPS spectrum of S2p scan of CdS-M photogenerated electron-hole pairs and consequently less reaction with trapped holes (Spanhel et al. 1987). Accordingly, the impact of initial pH on the photodissolution of CdS further supported the proposed mechanism of hole predomination.

DOM
The presence of DOM in natural waters (usually more than 2%) plays an important role in the fate and transformation of metal ions through direct complexation or oxidation with photogenerated ROS (Li et al. 2014;Pokhrel et al. 2014). To explore the effect of DOM on the photodissolution of CdS, experiments were conducted in the presence of 5 mg C/L HA and FA. A DOM-mediated inhibition of Cd 2+ release can be observed in Fig. 6b, especially in the presence of FA. The inhibition effect could be ascribed to three possible mechanisms: (i) HA and FA molecules could absorb the light energy due to their photosensitivity and then reduced the receipt of irradiation by CdS (Xue et al. 2021); (ii) the complex formed by DOM and Cd could control the stability of Cd (Jia et al. 2019); and (iii) HA and FA could also compete for the photons and holes, mitigating the oxidation dissolution of CdS. However, photoactive DOM can generate ROS, such as singlet oxygen ( 1 O 2 ), superoxide (•O 2 − ), and hydroxyl radical (•OH), which could promote the photooxidation of CdS Liu et al.2017). The overall effect of DOM depended on the interplay of the above mechanisms.

PO 4 3− and CO 3 2−
Phosphate and carbonate are well-known abundant anions in environmental media, which could affect the photooxidation process. Significantly inhibited release of Cd 2+ was observed during simulated solar irradiation with the addition of 10 mg/L PO 4 3− or CO 3 2− (Fig. 6c). Especially, in the presence of CO 3 2− , the concentration of Cd 2+ remained between 0.4 and 0.7 mg/L, much lower than that of the control. The main inhibition mechanism is the formation of insoluble and photostable Cd 3 (PO 4 ) 2 and CdCO 3 salts (K sp , Cd 3 (PO 4 ) 2 = 2.5 × 10 −33 and K sp , Cd CO 3 = 5.7 × 10 −13 ) (Jia et al. 2019), which cannot be oxidized by photogeneration holes or ROS in solution. Meanwhile, the solution pH value would increase correspondingly by the addition of PO 4 3− and CO 3 2− (Street et al. 1977;Gallant et al. 2006), generating cadmium hydroxide precipitation under alkaline conditions consequently affecting the photolysis of CdS (Luo et al. 2017;Matsumoto et al. 1996). Therefore, the passivation of the CdS surface by the cadmium salts or cadmium hydroxide precipitation leaded to the inhibited release of Cd 2+ . Phosphate and carbonate played a negative role in photodissolution of CdS.

Urea
Urea is the most commonly used nitrogen fertilizer in rice production in South China. Figure 6d showed that the release rate of Cd 2+ from CdS was markedly inhibited with the addition of 2 mmol/L urea. It has been documented that urea and N-containing compounds could be photodegraded in aqueous solutions under near-UV or visible radiation using the catalysts (such TiO 2 and so on), forming NH 4 + and NO 3 − ions (Makino et al. 1991;Nohara et al. 1997). Furthermore, the photocomposition of urea was mainly attributed to the generated •OH in the system as a strong oxidant. The competition of •OH appears to explain the inhibition effects of urea on the photodissolution of CdS. Therefore,

Photo-mediated transformation of CdS in soil under sunlight
In order to obtain further insight into the natural photodissolution process of CdS in soil, a solar irradiation experiment of paddy topsoil sample was performed on the campus of the Hunan Agricultural University from August 27, 2020 to September 10, 2020. The basic physical and chemical properties of the soil were introduced in Table 1. The soil pH is 5.98, and the total Cd content in the topsoil is 0.36 mg/kg, a concentration that exceeded that stipulated for "moderate contamination" by the Environmental Quality Standard of Soil guidelines of the People's Republic of China (0.30 mg/ kg, GB15618-2018). Figure 7a shows the concentrations of DTPA-extractable Cd in the topsoil sampled on days 3, 6, 9, 12, and 15. In natural solar irradiation group, the contents of extractable Cd in soil increased markedly with time; nevertheless, there were insignificant changes of extractable Cd under the dark condition. Meanwhile, the Cd 2+ concentrations in surface water increased significantly with illumination time under solar condition, while the increase of Cd 2+ was insignificant under dark (Fig. 7b). During the 15 days of irradiation, the concentrations of DTPA-extractable Cd in soil increased from 0.81 to 2.18 mg/kg. The above results indicate that sunlight plays an important role in the mobility and bioavailability of Cd in natural soil even in waterlogged paddy soil, which might be attributed to the photodissolution of CdS in topsoils. Furthermore, the soil pH increased, and Eh decreased in both of the light and dark groups, the first day been taken as a control (Fig. S1). These changes could be related to the waterlogging treatment, and similar results have been reported in the previous study .

Distribution of Cd fractions in soils
The distributions of Cd among different fractions in the soil are presented in Fig. 8. The acid-extractable fraction (F1) Cd has the greatest mobility and bioavailability, whereas the residual fraction (F4) Cd is the most stable form and was unavailable to biota. The reducible fraction (F2) mainly consists of Cd bound to amorphous Fe-Mn oxides and hydroxides, whereas the oxidizable fraction (F3) Cd is predominantly associated with organic matter and sulfides (Wang et al. 2021). As shown in Fig. 8, acid-extractable Cd was the largest fraction, followed by the oxidizable, residual, and reducible fraction. F1 accounted for 42.49% and 34.97% of Cd in light group and dark group soil samples, respectively, compared to 38.92% in initial soil. The proportions of the oxidizable and residual Cd fractions in three soil samples were similar. Notably, the reducible fraction in light group (20.76%) was lower than that in the initial soil (23.45%), indicating the transformation of Cd associated with organic matter and sulfides to other fractions in soils under natural sunlight. These results showed that the Cd fractions were affected by solar irradiation, as the acid-extractable Cd increased, whereas the oxidizable Cd decreased in the light group. It was suggested that natural sunlight can activate soil Cd and increase its bioavailability and environmental risk. Moreover, the waterlogging treatment can reduce the Cd mobility in soil, consistent with previous observations ).

Conclusions
CdS is a stable form of Cd in natural soil environments, which belongs to the oxidizable fraction. We herein investigated the effect of the sunlight on the Cd stability and activity. The kinetic experiments showed that simulated solar irradiation enhanced the release of Cd 2+ from CdS-M significantly. The Cd 2+ release rate increased from pH 3.98 to pH 6.00, while drastically decreased as the initial pH increased to 8.11. In addition, all of the co-existing DOM, PO 4 3− , CO 3 2− , and urea negatively affected the photodissolution of CdS in solution. The possible mechanism of the solar-mediated Cd 2+ release from CdS was predominated by the holes; besides, the photogenerated •O 2 − and •OH were also possibly involved in the photo-induced reaction. The soil illumination experiment results showed that natural sunlight can enhance the concentration of extractable Cd, which is more easily absorbed by organisms, resulting in the increase of environmental risk. These findings indicate that the solar irradiation may play an important role in shaping Cd species, to increase its mobility and bioavailability in natural soils. However, the irradiation exposure was only performed in laboratory settings under simulated conditions. The complexity of the environmental medium calls for studies under field or field-like conditions.