Baogui Li,Yuan Liu,Zhen Tao,Zhijuan Zhao,Tao Fan,Zhongyang Li
Sorption and mobility of cadmium in soil impacted by irrigation waters
Cadmium,Soil,Reclaimed water irrigation,Livestock wastewater irrigation,Sorption,Desorption
Soil contamination by Cd has drawn global attention, while how irrigation waters modulate Cd sorption and mobility in soil remains obscure. We address this by investigating how cropped sandy soil irrigated with different waters altered Cd sorption and mobility using a rhizobox experiment followed by a batch experiment. Maize were planted in the rhizoboxes and irrigated by reclaimed water (RW), livestock wastewater (LW) and deionized water (CK), respectively. The bulk soil sampled from each treatment after 60 days of growth was employed to measure the Cd sorption and mobility using the isothermal adsorption and desorption experiments. The results showed that, in a small rhizobox experiment, the adsorption rate of Cd by the bulk soil in the adsorption phase was much faster than the desorption rate in desorption phase. Irrigation with RW and LW both reduced the Cd adsorption capacity of soil, and the reducing degree brought by LW was more obvious. Cd desorption rate was very low but keep increasing in the desorption stage, and pre-RW irrigation had the potential to increase Cd desorption from soil. Although the results were obtained based on the bulk soil sampled from a rhizobox experiment, our study strongly suggests that the altered Cd adsorption and desorption behavior in the soil caused by the RW and LW irrigation may risk the farmland ecosystem and deserve more concern.
Sorption and mobility of cadmium in soil impacted by irrigation waters

Soil contamination by Cd has drawn global attention, while how irrigation waters modulate Cd sorption and mobility in soil remains obscure. We address this by investigating how cropped sandy soil irrigated with different waters altered Cd sorption and mobility using a rhizobox experiment followed by a batch experiment. Maize were planted in the rhizoboxes and irrigated by reclaimed water (RW), livestock wastewater (LW) and deionized water (CK), respectively. The bulk soil sampled from each treatment after 60 days of growth was employed to measure the Cd sorption and mobility using the isothermal adsorption and desorption experiments. The results showed that, in a small rhizobox experiment, the adsorption rate of Cd by the bulk soil in the adsorption phase was much faster than the desorption rate in desorption phase. Irrigation with RW and LW both reduced the Cd adsorption capacity of soil, and the reducing degree brought by LW was more obvious. Cd desorption rate was very low but keep increasing in the desorption stage, and pre-RW irrigation had the potential to increase Cd desorption from soil. Although the results were obtained based on the bulk soil sampled from a rhizobox experiment, our study strongly suggests that the altered Cd adsorption and desorption behavior in the soil caused by the RW and LW irrigation may risk the farmland ecosystem and deserve more concern.

Cd accumulation in soil induced by anthropogenic activities is a global environmental concern over the past two decades activities because of its toxicity to human health and detrimental impact on plant growth and soil functions [1]. Remediation of Cd-contaminated soil is crucial for food safety and sustainable agriculture, and requires a thorough understanding of the mobility and bioavailability of soil Cd. The bioavailability and mobility of Cd in agricultural soil are affected by agronomical practices like fertilization, irrigation, cropping and so forth [[2], [3], [4]]. Previous studies reported that soil additives (e.g. biochar, hydroxyapatite, humic acid, zeolite) are important factors that influence Cd mobility and bioavailability as they affect soil pH, ion strength, organic matter, attendant cations or anions, functional groups on the sorbent [[5], [6], [7], [8]]. Water management plays a critical role in soil Cd bioavailability and mobility [[9], [10], [11]]. It was reported that the pore water velocity under flood irrigation affected Cd transport in a sandy loam soil with high or low salinity based on column experiments [12]. Irrigation with treated wastewater only raised the metals (Zn and Cd) sorption ability of calcareous soils relative to that with the blending of treated wastewater and surface water [13]. Therefore, it is necessary to intensively study the influential mechanisms of water management on soil Cd mobility and bioavailability. Due to the shortage of water in arid and semiarid regions, reclaimed water (RW) and even the raw wastewater were used for irrigation, which might not only aggravate soil Cd contamination, but also increase the bioavailability and mobility of Cd in soil. Generally, the content of N, P, chemical oxygen demand (COD), Cu, Zn and antibiotics is higher in livestock wastewater (LW) [[14], [15], [16]], while the electrical conductivity and the content of salt ions are higher in RW [17]. As a result, the properties of irrigated soil by RW and LW differed from the soil irrigated by clean water and hence altered the bioavailability and mobility of soil Cd. Unfortunately, the influencing mechanisms of irrigation with wastewater and reclaimed water on soil Cd mobility and bioavailability have not yet been studied sufficiently. In particular, the sorption and desorption of Cd in the soil irrigated with different unconventional water sources should be researched intensively to gain an insight knowledge of Cd migration at water and soil interface. The mechanisms regarding Cd immobilization involves electrostatic attraction, cation exchange, complexation and precipitation [18]. These processes were impacted by the interaction between soil properties and Cd [19]. We hypothesized that irrigation with RW and LW results in the changes in soil properties, and hence alters the Cd(II) fixation ability of soil. To test this hypothesis, batch adsorption and desorption equilibrium experiments were employed in this study. We aimed to: (1) compare the sorption and desorption of Cd in reclaimed water- and livestock wastewater-irrigated soils with clean water-irrigated or unirrigated soils; and (2) to clarify the dominate factors controlling the distinction in retention capacity of Cd in these soils.

The soil used in the experiment was taken from the rhizobox experiment in our previous work under irrigation with different waters [20]. The soil, originated from alluvial sediments of Yellow River with a silt loam texture (sand:silt:clay = 15:79:6), is classified as Calcaric Fluvisols according to World Reference Base (WRB) [21]. The details of the rhizobox experiment were given in Ref. [20], but for completeness it was briefly explained here. The soil was taken from the top 20 cm in an experimental station irrigated with groundwater at Xinxiang, Henan province of China. The soil was air-dried and ground first, and it was then thoroughly mixed with base fertilizer consisting of urea (200 mg N kg−1), calcium superphosphate (100 mg P kg−1) and potassium chloride (200 mg K kg−1) prior to being packed into 14 cm × 12 cm × 17 cm rhizoboxes. Each rhizobox consisted of five compartments (5:1:2:1:5) along its 14 cm side by 48 μm nylon mesh and was filled with 3 kg of soil. Three maize seeds of Jundan 20 variety were drilled into the central compartment. The rhizoboxes were irrigated with reclaimed water, livestock wastewater and deionized water (DW, control). The reclaimed water was taken from Camel Bay sewage treatment plant in Xinxiang and the livestock wastewater from the anaerobic fermentation tank at Xinxiang Shengda Animal Husbandry Co., Ltd. The water properties are given in Table S1. Prior to its use for irrigation, the livestock wastewater was diluted with deionized water at a 1:5 ratio. Each treatment has three replicates and all experiments were conducted in a greenhouse. The maize was harvested 60 days after seedling emergence. To exclude the root-induced impact, we employed the soil from the end compartment far away from the central rhizosphere for sorption and desorption analysis. As a comparison, the original and unplanted soil (OS) were also recruited in the analysis. All the air-dried soils were ground and passed through a 0.25 mm sieve prior to the sorption and desorption experiments, and the physical and chemical properties of these soils were given in Table S2. The available Cd in the soils was at similar level. The nutrients and salt content in the soil irrigated with RW and LW was higher than that with DW. RW-irrigated soil exhibited high electrical conductivity (EC), pH and soluble Na+, Cl−, HCO3− and SO42− but low organic matter (OM), soluble K+, Ca2+ and NO3−, available Cu and available Zn, compared to LW-irrigated soil.

The soil of 3 g was added to a 50-mL polyethylene centrifuge tube, and followed by the blend with 30 mL of mixed solution containing 0.01 M NaNO3 solution as a supporting electrolyte and Cd (II) with a specific concentration (5, 10, 20, 40 or 50 mg L−1) made by Cd(NO3)2·4H2O reagent. When 0.01 mol L−1 NaNO3 solution with no Cd(II) (0 mg L−1) was used [22], Cd was almost undetectable in the solution during the whole adsorption and desorption procedures (Fig. S1), hence the effect of Cd release by the soil itself on the sorption and desorption processes in this study was regarded as negligible. After the tube was shaken at 200 rpm for 24 h at 25 ± 1 °C followed by 10 m of centrifugation at 4000 rpm, 5 mL of supernatant was pipetted out for Cd concentration analysis. Then the tube was shaken for a further 96 h, and the supernatant was also drawn for Cd concentration measurement after centrifugation. To monitor the pH of the system, the pH in the suspension with initial Cd(II) of 20 mg L−1 was measured using a pH meter (Orion-star A211, USA) with a pH electrode (Thermo Orion 9157BNMD, USA) during the 120 h. The concentration of Cd(II) in the supernatant was measured by an atomic adsorption spectrometry (AAS) (AA-6300, SHIMADZU, Japan), and the amount of Cd adsorbed was calculated from the following formulae:where and are Cd adsorbed by the soil at 24 h and 120 h respectively after the onset of the experiment, C0 is the concentration of Cd(II) in the prepared solution, and C1 and C2 are the concentration of Cd(II) in the supernatants at 24 h and 120 h after onset of experiment respectively (mg L−1), V0 (30 mL) is the volume of prepared solution added to the tube (30 mL), V1 (25 mL) is the volume of solution remained in the tube after the first supernatant sampling, m (3 g) was the mass of the soil.

Na+ and NO3− exerted less effect on Cd(II) adsorption compared with other cations and anions [23], and NaNO3 is commonly used as the background electrolyte for the adsorption batch experiments, hence we also choose NaNO3 as the background electrolyte in our study. By the way, the NaNO3 concentration of 0.01 mol L−1 have been recommended by tremendous of studies [24,25]. The soil and the residual solution with the tube were weighed immediately after all the supernatant was carefully poured out at the end of the adsorption experiment. To begin the desorption process, NaNO3 solution (30 mL, 0.01 mol L−1) was added, followed by 24 h of shaking at 200 rpm. Then a 5-mL aliquot of supernatant was sampled for Cd(II) analysis and the rest was gently emptied as mentioned previously. Then the tube with the residuals was weighed and the above procedures were repeated four times to measure how much Cd could mobilize. The mobilized Cd was calculated as follows:where is the Cd released from soil after adsorption at different sampling time (mg kg−1), is the Cd(II) concentration in the solution of the jth desorption (mg L−1), is the Cd(II) concentration in the solution at the end of adsorption (j = 1) or the Cd(II) concentration in the solution of the (j-1)th desorption (j = 2, 3, 4) (mg L−1), is the total weight of dry soil and tube (g), are the total weight of solution residue, soil and tube before the jth addition of desorption solution, n2 is the times of desorption solution added. The desorption ratio was calculated as follows:where is all Cd adsorbed by the soil measured in the adsorption process.

The adsorption isotherm was described by the following Langmuir formula [26]:where Sads is the adsorbed Cd by soil when the Cd(II) concentration in the equilibrium solution is , is distribution coefficient, and is the maximum Cd the soil could adsorb. As a comparison, the adsorption result was fitted to the Freundlich formula [27]:where is the distribution coefficient and b is a fitting parameter.

The difference between treatments was assessed by the analysis of variance (ANOVA) using IBM SPSS 25, and the Duncan's multiple range test was used for post-hoc pairwise comparisons at significant level (p < 0.05). Pearson correlation coefficients were used to analyze the correlations between the adsorption and desorption of Cd and soil physicochemical characteristics.

In the adsorption process, the pH reading of the reaction system shown by the pH meter increased rapidly in the first 20 min, then changed slightly between 20 min and 120 h (Fig. 1). The increase in adsorbed Cd with the initial Cd (II) concentration between different treatments were compared after 24 h and 120 h of the adsorption experiment (Fig. 2). After 24 h, more than 99.6% of Cd in the solution was immobilized, indicating that the soils were very sorptive to Cd (Fig. 2A). After 24-h reaction, the remaining Cd in equilibrium solutions in the irrigated soils was not significantly altered relative to the original soil with one exception. After 120 h, the remaining Cd in the equilibrium solution of original soil treated with 40 and 50 mg L−1 Cd was the lowest (Fig. 2B), indicating that the original soil was the most sorptive to Cd. Cd concentration in the equilibrium solution of the soil irrigated with deionized water was basically higher than that of the original soil, when they were treated by 5, 20, 40 and 50 mg L−1 Cd. The Cd concentration was the highest in the equilibrium solutions of the soil irrigated by LW when the initial Cd was >10 mg L−1, demonstrating that the Cd mobility was the highest in the soil irrigated LW. The simulation results of Freundlich and Langmuir equations matched the nonlinear Cd isotherms well (Fig. 3). A set of calculated parameters of the equations are listed in Table 1. All the correlation coefficients (r2) in the Freundlich equations are above 0.996 and higher than those in the Langmuir equations, indicating that Freundlich equations fit the Cd(II) adsorption isotherms in these soils better. Some previous studies [28,29] also confirmed this, probably because the Langmuir equation neglected the lateral interactions and horizontal mobility of the adsorbed ions [30]. All values of b in this study ranged from 0.605 to 0.765 after 24-h and 120-h adsorption periods. Parameter b is a dimensionless parameter commonly less than 1 [31], and also a measure of the heterogeneity extent in sorption sites in equilibrium batch experiments and represents different affinities for Cd(II) by matrix surface. Specifically, the b values of these irrigated soils decreased from the initial 24-h sorption period to the finial 120 h sorption, meaning that the heterogeneity extent in sorption sites of soils was altered after a longer reaction time.

With the increase of desorption time, the amount of Cd(II) desorbed from these soils increased (Fig. 4). The desorption amount of Cd(II) after 120 h was only 0.81–0.18 mg kg−1, and 13.1–19.0 mg kg−1 in the soils treated by 5 and 50 mg L−1 of the initial Cd(II), respectively (Fig. 4A–E). In most cases, more Cd(II) was desorbed from the soil which sorbed more Cd at the end of sorption procedure. After 120 h of desorption, the desorption of Cd(II) was significantly higher in the soil irrigated by RW than that in the original soil at the initial sorption concentration of 20 mg L−1 (Fig. 4C); no significant difference of Cd desorption was observed between the soil irrigated with LW and the original soil; Cd(II) desorption was significantly lower in the soil irrigated by DW than the original soil at 50 mg L−1 of initial sorption Cd(II) (Fig. 4E); the desorption of Cd(II) was significantly higher in the soil irrigated with RW than that in the soils irrigated with LW and DW at the initial Cd sorption concentration of 10, 20 and 50 mg L−1 (Fig. 4A, C and 4E). There was no significant difference between the treatments irrigated with LW and DW irrespective of the initial Cd(II) sorption concentrations at the end of the five round of desorption. The change in desorption ratio with time (Fig. 5) was similar to that of desorption amount. The total desorption ratio of Cd(II) in these soils was 0.14%–0.50% at 24 h and 1.62%–3.86% at 120 h (Fig. 5A and E), indicating that most of the adsorbed Cd(II) was firmly immobilized by these soils, and that the desorption did not reach the equilibrium even at 120 h. At 5 mg L−1 of initial Cd(II), the desorption ratio of Cd(II) in the soil irrigated with DW was the highest followed by that in the soil irrigated with LW (Fig. 5A). At 10–50 mg L−1 of initial Cd(II), the desorption ratio of Cd(II) in the soil irrigated with RW was the highest (Fig. 5B–E). The desorption ratio of Cd(II) in the soil irrigated with LW was evidently lower than that in original soil at 10 and 50 mg L−1 of initial Cd(II) (Fig. 5B and E).

The associations between the adsorbed Cd(II) by soil and the soil properties were presented in Table 2. When the initial Cd(II) concentrations were 40 and 50 mg L−1 at the beginning of the adsorption, the Cd adsorption amount was significantly positively associated with the soil pH, and negatively with available P and water soluble Ca2+. At the initial Cd(II) of 20 mg L−1, water soluble Ca2+ was notably negatively with the adsorbed Cd(II) by soil. At the initial Cd(II) of 10 mg L−1, carbonate and available Cu showed the significant negative correlations with the soil-sorbed Cd.

This study compared the Cd adsorption and desorption carried out in a batch experiment between bulk soils collected from a rhizobox experiment pre-irrigated with RW, LW and DW and the original unirrigated soils at a range of initial Cd sorption concentrations. As we hypothesized, Cd fixation ability of soil was altered by the irrigations. The soils used in this study were potent to fix the Cd in solution. The sorption of Cd(II) by these soils was time- and concentration-dependent. The study with batch sorption-desorption experiments also found that the potential of soils to retain Cd were highly dependent on soil properties as well as the initial Cd concentration [19]. For the high-initial Cd(II) systems after 120-h sorption, the fertilized and deionized water-irrigated soils showed decreased capability of immobilizing Cd compared with the original soil, and the soils irrigated with livestock wastewater possessed the lowest Cd fixation capability. After 120 h of desorption, reclaimed water-irrigated soils desorbed more Cd than other treatments when the initial Cd was more than 5 mg L−1.

Irrigation undoubtedly led to an alteration of soil physical and chemical properties, thereby controlling Cd(II) mobility and retention in soils [32]. The soil characteristics control Cd sorption via the mutual effects of pH, the combination of precipitation with other anions in soil solution, and the complexion with organic matter, among others [18,29]. In this study, soil pH was positively correlated with the Cd(II) sorption amount (Table 2). Batch sorption experiment also showed that Cd sorption capacity of soils is highly related to soil physicochemical properties, especially controlled by soil pH [33]. The pH influences the ionic state of Cd and the adsorbent functional groups, as well as the surface charge of adsorbent. When the pH is less than 8 (e.g. soil irrigated with LW), free Cd2+ cations are the main species in the aqueous solutions; while when the pH value is between 8 and 8.3 (the original soil and soil irrigated with RW and DW), Cd(OH)+ and Cd(OH)2 were the coexisting forms of Cd(II) in addition to the main Cd2+ form [34]. Therefore, the precipitation of Cd(OH)2 was higher in RS than LS. Moreover, it is well-known that the adsorption quantity of Cd(II) increases with the increasing initial pH of solution due to the decreased competition by H+ and the increased negative charge on soil surface [35]. OM was also an important factor affecting behavior of metals. Soil OM exhibits high cation exchange capacity (CEC) values and abundant functional groups, thus providing more adsorption sites and facilitating Cd(II) adsorption [23]. While it is also reported that dissolved OM could form ligand-like complexes and reduce Cd(II) adsorption [36], which could partly explain that soil irrigated with dissolved OM-enriched livestock wastewater, adsorbed less Cd(II) in our study. Besides, soil texture is also important influence factor to retain Cd with different fraction [37], possibly more sand and silt in LW-irrigated soil resulted in the less soil surface area, thereby providing fewer adsorption sites for Cd(II) compared with RW- and DW-irrigated soil. The cations in the soil play an essential role in Cd(II) adsorption and desorption, and the increase in cation content is adverse to the sorption of heavy metals [38]. The Cd sorption and retention in soils highly depend on the competition with other heavy metals for the sorption sites. Ca2+, Mg2+, Cu2+, and Zn2+ can be introduced into soils with previous fertilizers and later irrigation with these treated wastewaters, and thus occupied the sorption sites or formed the complexation with other anions to prevent the Cd sorption in soils [35]. The total content of water soluble K+, Na+, Ca2+, Mg2+ was higher in LW-irrigated soil (617.7 mg kg−1) than RW-irrigated soil (557.0 mg kg−1) and DW-irrigated soil (413.1 mg kg−1), hence the Cd sorption amount was lower in LW-irrigated soil. In the similar way, the bioavailable Cu and Zn were negatively related with Cd adsorption amount (Table 2) because their higher content in LW-irrigated soil promoted the competitive sorption of these metal cations with Cd, which is consistent with previous studies [[39], [40], [41]]. The promoting effect of water soluble Na+ and Mg+ on the release of Cd(II) by the soils after adsorption was also observed (Table S3). The anions are also nonnegligible factors controlling Cd(II) adsorption and desorption. For example, NO3−, SO42−, and Cl− could form mononuclear or polynuclear complexes with metal ions in solution and these complexes may carry a positive, negative or zero (neutral) charge depending on the charge and the number of anions involved, which consequently affecting Cd(II) adsorption [42]. It was found that Cl− would form with Cd compounds whereas NO3− and SO42− have difficulty forming the complexes with Cd2+ in batch adsorption experiment [43,44]. In our study, the NO3− content was higher in LW-irrigated soil than other soils, and total content of Cl− and SO42− was higher in RW-irrigated soil. The effect of Cl− and SO42− anions on Cd(II) adsorption was higher than NO3−, and Cl− was proved to form more specific sorption on the soil surface relative to NO3− and to occupy more ionic exchange sites [23]. The complexation of Cl− and SO42− with Cd(II) during the desorption stage enhanced the Cd release, which may explain the positive associations between the desorbed Cd and these two anions (Table S3) and the higher desorption of Cd in LW-irrigated soil. The content of PO43− and HCO3−, which could form precipitates and complexation with Cd(II) [45], were higher in RW-irrigated soil than LW-irrigated soil, thereby contributing to more immobilization of Cd in RW-irrigated soil. Therefore, Cd sorption in soils is complex and can be affected by the type of anions present in the soils, further deciphering the effects of anions on Cd sorption in soils is important to elucidate the mechanisms involved in the Cd migration under treated wastewater irrigation. What's more, the emerging contaminate antibiotics were always found with higher concentration in livestock wastewater (Table S1) and the corresponding irrigated soils [46], and our soils were no exception. The positively charged groups of antibiotics could compete with Cd(II) for adsorption sites [47], simultaneously the negatively charged groups of antibiotics could form complexes with Cd(II) and be in favor of the mobility of Cd(II). In addition, the antibiotics and metals could be adsorbed on the soil surface by antibiotics-Cd-soil bridging [48,49]. If the ternary complex is more stable than the dissolved Cd-antibiotics complex, the sorption of metals will be enhanced [25]. Therefore, the effect of antibiotics on Cd(II) adsorption is the combination of above processes, and in our study perhaps the antibiotics competition in LW-irrigated soil and the enhanced mobility of Cd-antibiotics complexes were the more powerful processes which inhibited the adsorption of Cd(II) by LW-irrigated soil relative to RW-irrigated soil.

Cd sorption in soils was found to establish rapid equilibrium followed by a pseudo-second order reaction, the contact time between Cd and soils has a great effect on Cd desorption period [50,51] Adsorption of Cd(II) was a complex process, including a quick reaction in which Cd(II) adsorbed onto the exterior surface of soil colloids and a slow reaction that Cd(II) moved to the interior surface [52], which is also observed from the pH dynamic in this study (Fig. 1). The first quick and the following slow pH change indicated Cd(II) adsorption was mainly driven by chemical adsorption. At the initial stage of adsorption, soil surface provided more adsorption sites, which contributed to the rapidly increased adsorption rate. The negative-charged soil surface could combine with metal cations through Coulomb force, and this process could be considered as non-specific adsorption. Thus, the adsorbed Cd(II) could be easily desorbed from the soil surface by external disturbance in this process. Then, the slow specific adsorption happened through complexation between Cd(II) and soil surface, and it would increase the positive-charged sites of soil surface, which would in turn prevent adsorption of Cd(II), hence it was observed that the Cd(II) concentration increased (Fig. S1) and pH decreased (Fig. 1) in the solution at 120 h compared to that at 24 h. Additionally, when the adsorption amount was increasing, the decreased Cd(II) concentration in solution and available sorption sites on soil surface became limiting factors for Cd(II) adsorption [53].

The concentration of Cd in soil solution is a critical factor that affects the Cd adsorption in soils. The primary and dominant mechanism of interaction between soil particles and Cd is sorption, which means that Cd can be adsorbed onto the soil surface by electrostatic forces. However, at high concentrations of Cd in soil solution, the Cd sorption capacity may become saturated with the form of solution precipitation and soil particle surface [54]. At low concentration (5 mg L−1) of initial Cd(II) in this study, almost all the Cd in the solution could be adsorbed by these soils, therefore a rather small amount of Cd was left in the solution, and no remarkable distinction in the residual Cd occurred between these soils. In other words, when the soil like the one in our study was subjected to the rainwater or the stormwater containing low-level Cd resulted from the heavy air pollution or flood invasion, the adsorption sites on the soil surface were adequate for fixing almost all the enthetic Cd, hence the risk of remaining Cd was low for the soil. At higher concentration (10, 20, 40 and 50 mg L−1) of initial Cd(II), the adsorption sites of soil didn't seem enough to immobilize all the solution Cd. With the increase of the initial Cd concentration, the Cd left in the equilibrium solution increased and varies with different soils. Although the risk increased with high-level of exogenous Cd, the hazard of Cd in soil irrigated with RW was lower compared with that with LW. It is reported that chemisorption (cation exchange) and co-precipitation were the dominate mechanisms at low and high initial Cd(II) loadings respectively [55]. Consequently, the different mechanisms of Cd(II) sorption at low and high Cd loadings might contribute to the discrepancy of the order of Cd(II) adsorption amount among these soils. Another possibility is that the stability field of the Cd species depending on Cd(II) concentrations altered the affinity of different Cd species in these irrigated soils [54].

Sodium nitrate was commonly used to desorb the exchangeable fraction of metals in previous studies [24,25]. Soil exhibits various affinities to retain Cd as indicated by its reaction rates during sorption processes, which is similar to the desorption rate for Cd release. The desorption ratio is rather low in our study, and the desorption amount kept increasing during the 120-h desorption, suggesting that the release of Cd was a slow process without reaching the equilibrium even after 120 h of desorption, and that a large amount was retained in the soil phase. In this experiment, the exchangeable fraction with high availability only accounted for a small proportion compared to the less mobile complexed fraction, which may explain why the amount of Cd(II) desorption by 0.01 mol L−1 NaNO3 was very insignificant in our study. The carbonates and other soil fraction including Fe/Mn oxide might also strongly bound Cd [19,56] and the bounded Cd was very difficult to be desorbed by NaNO3. Distinguishing the different forms (exchangeable fraction, carbonate-bound fraction, easily reducible Mn and Fe oxide fraction, organic bound fraction and residual fraction) of Cd adsorbed on the soil by successive extractions using a series of desorbents may shed more light on the mechanism of Cd adsorption. Overall, the soil used in this study possessed a strong adsorption capacity for Cd, and we guess that the influence of soil property differences on its adsorption capacity is far greater than that of irrigation, which needs further evaluation.

Given that more Cd was left in LS solution in adsorption stage at high-initial Cd scenarios, the risk of LW irrigation was higher compared to other treatments, the residual Cd (0.01–0.76 mg L−1) might risk plant growth according to the concentrations of cadmium toxic to plants (Table S4). In desorption stage, reclaimed water irrigation made soil desorb more Cd than other irrigated soils, and the desorption continued even after 120 h, indicating that the desorption of Cd in these soils would possibly cause more serious phytotoxicity and deserves more concern. For the clean water-irrigated soil, reclaimed water or livestock wastewater irrigation reduce the Cd retention by soil, which is unexpected by us. For the Cd-polluted soil, the Cd mobility may also be enhanced after irrigation with unconventional water sources which needs to be clarified in future studies.

The effect of reclaimed water and livestock wastewater irrigation on soil ability in Cd(II) sorption at different initial Cd(II) concentrations with the controls of deionized water irrigation and the original soil before irrigation was investigated via a rhizobox experiment followed by a batch experiment. Reclaimed water and livestock wastewater irrigation notably influenced soil physicochemical properties, resulting in a different sorption ability of Cd(II) in the associated irrigated soils. With respect to the low initial concentration (5 mg L−1), there were no remarkable differences between sorption of Cd(II) in these soils, especially after 24-h adsorption. However, after 120-h reaction, the Cd adsorption amount of these soils varied with each other obviously with the notable adsorption decline in these irrigated soils particularly for livestock wastewater-irrigated soils. Freundlich model fitted the Cd(II) sorption isotherms in these irrigated soils better than Langmuir model and the maximum sorption of Cd(II) of these irrigated soils for 120 h simulated by both models was lower than that for 24 h. Moreover, a small proportion but continuous release of the sorbed Cd(II) was observed in the desorption stage. In conclusion, the risk of pre-livestock wastewater irrigation was higher than pre-reclaimed water irrigation due to the resulting lower fixation ability of Cd(II) of the irrigated soil, and the incessant release of Cd from soil needs more concern. This study suggests that the risk of livestock wastewater irrigation was higher compared to reclaimed water regarding the Cd migration, and the long-term monitoring of Cd release is also essential for the alkaline sandy soil irrespective of the irrigation waters. It needs to be emphasized that the results could be varied when these tests are carried out in the contaminated soils in the field.

Zhongyang Li: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper. Yuan Liu: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper. Baogui Li: Performed the experiments; Analyzed and interpreted the data; Wrote the paper. Zhen Tao: Performed the experiments; Analyzed and interpreted the data. Zhijuan Zhao; Tao Fan: Performed the experiments; Contributed reagents, materials, analysis tools or data.

Data included in article/supp. Material/referenced in article.

Supplementary Materials: Table S1: Basic properties of reclaimed water and livestock wastewater; Table S2: Properties of original soil (OS), deionized water-irrigated soil (DS), livestock wastewater-irrigated soil (LS) and reclaimed water-irrigated soil (RS) used in this study; Table S3: Pearson correlation coefficients presenting the correlations between soil-desorbed Cd(II) after reaction for 120 h and soil physicochemical characteristics; Table S4: Summary of cadmium concentration range which can cause phytotoxicity; Figure S1: Adsorption isotherms of Cd on different treated soils within a reaction time of 24 h and 120 h, respectively.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper