A Sunflower and Dolomitic Lime Experiment to Modify Cadmium Concentration in Cacao

Cadmium contamination in cacao farms of Piura, North Peru: A comprehensive assessment of geogenic and anthropogenic sources and implications for future production

Cadmium (Cd) is a highly mobile and toxic heavy metal that negatively affects plants, soil biota, animals and humans, even in very low concentrations. Currently, Cd contamination of cocoa produced in Latin American countries is a significant problem, as concentrations can exceed acceptable levels set by the European Union (0.5 mg/kg), sometimes by more than 10 times allowable levels. In South America, Theobroma cacao is an essential component of the basic market basket. This crop contributes to the Latin-American trade balance, as these countries export cacao and chocolate-based products to major consumer countries such as the United States and Europe. Some soil amendments can alter the bioavailability and uptake of Cd into edible plant tissues, though cacao plants can accumulate Cd without displaying any visible symptoms of phytotoxicity, which makes it difficult to determine if potential remediation strategies are successful. Currently, the only effective way to quantify Cd accumulation in plant tissues is via destructive post-harvest practices that are time-consuming and expensive. New hyperspectral imaging (HSI) technologies developed for use in high-throughput plant phenotyping are powerful tools for monitoring environmental stress and predicting the nutritional status in plants. Consequently, the experiments described in this thesis were conducted to determine if HSI technologies could be adapted for monitoring plant stress caused by Cd, and estimating its concentration in vegetative plant tissues. Two leafy green crops were used in these experiments, basil (Ocimum basilicum L. var. Genovese) and kale (Brassica oleracea L. var. Lacinato), because they are fast growing, and therefore, could serve as indicator crops on cacao farms. In addition, we expected these two leafy green crops would differ in their morphological responses to Cd stress. Specifically, we predicted that stress responses would be visible in basil, but not kale, which is known to be a hyperaccumulator. The plants were subject to four levels of soil Cd (0, 5, 10 and 15 ppm), and half of the pots were amended with biochar at a rate of 3% (v/v), as this amendment has previously been demonstrated to improve plant health and reduce Cd uptake. The experiments were conducted at Purdue’s new Controlled Environment Phenotyping Center (CEPF). The plants were imaged weekly and manual measurements of plant growth and development were collected at the same times, and concentrations of Cd as well as many other elements were determined after harvest. Fourteen vegetation indices generated using HSI images collected from the side and top view of plants were evaluated for their potential to identify subtle signs of plant stress due soil Cd and the biochar amendment. In addition, three mathematical models were evaluated for their potential to estimate Cd concentrations in the plant biomass and determine if they exceed safe standards (0.28 mg/kg) set by the Food and Agriculture Organization (FAO) for leafy greens. Results of these studies confirm that like many plants, these leafy green crops can accumulate Cd levels that are well above safety thresholds for human health, but exhibit few visible symptoms of stress. The normalized difference vegetation index (NDVI) and the chlorophyll index at the red edge (CI_RE) were the best indices for detecting Cd stress in these crops, and the plant senescence and reflectance index (PSRI) and anthocyanin reflectance index (ARI) were the best at detecting subtle changes in plant physiology due to the biochar amendment. The heavy metal stress index (HMSSI), developed exclusively for detecting heavy metal stress, was only able to detect Cd stress in basil when images were taken from the top view. Results of the mathematical models indicated that principal components analysis (PCA) and partial least squares (PLS) models overfit despite efforts to transform the data, indicating that they are not capable of predicting Cd concentrations in these crops at these levels. However, the artificial neural networks (ANN) was able to predict whether leafy greens had levels of Cd that were above or below critical thresholds suggested by the FAO, indicating that HSI could be further developed to predict Cd concentrations in plant tissues. Further research conducted in the field and in the presence of other environmental stress factors are needed to confirm the utility of these tools, and determine whether they can be adapted to monitor Cd uptake in cacao plants.

Regulating soil pH becomes a crucial practice to alleviate cadmium (Cd) contamination. However, little is known about the threshold of soil pH for the safe production of rice at various soil Cd levels. In this paper, the relationships between soil pH values and the contents of available Cd extracted by calcium chloride (CaCl2-Cd) in neutral and acidic soils were studied by mandatory acidification with H+ addition or neutralization with lime at various soil Cd levels. The results showed that the soil CaCl2-Cd contents dramatically decreased with increasing soil pH, and a logarithmic function could well describe the relations of soil CaCl2-Cd contents and soil pH at constant total Cd (CaCl2-Cd model). The Cd contents in rice grain (grain-Cd) in relation to soil CaCl2-Cd was further established through modified rice pot experiments. A model for the prediction of Cd content in rice grains (grain-Cd model) was set up, though which the grain-Cd content could be predicted based on soil pH and total Cd content. 122 data pairs of rice grain-Cd contents obtained at various soil total Cd contents and pH were employed from the literature to verify the reliability of the established model, approximately 95.08% of those data favorably located within the 1:1 line ± 0.5 unit area of the grain-Cd model. Notably, this model can be applied to determine the thresholds of soil pH at a specific Cd pollution level. For instance, to achieve a rice grain-Cd contents matching the Chinese national food safety limit of 0.2mgkg-1, the soil pH thresholds were estimated to be 5.05, 5.70, and 6.02 at soil Cd contents of 0.3, 0.6, and 0.8mgkg-1, respectively. In addition, the established model can also be used to estimate the health risk from rice in broad regions with various soil pH values and Cd contents.

Wetland macrophytes have advantages when used in the remediation of cadmium (Cd)-contaminated paddy fields because they can adapt to overly wet soil environments; however, only a few studies have tested the efficiency of macrophytes in Cd phytoremediation. In this study, we investigated the effect of soil pH (pHs of 5, 6, and 7) on the accumulation and translocation of Cd by Polygonum hydropiper (L.) in low and moderately Cd-contaminated paddy soil (0.56 and 0.92 mg/kg, respectively). Our results indicated that Cd accumulation in stems and roots, as well as subcellular distribution in P. hydropiper, was affected by soil pH, with significant interactions between the soil pH and Cd level. At low soil Cd levels, stem and root Cd contents were higher at a soil pH value of 6. In addition, with higher soil pH values, the proportion of Cd distributed in the cell wall increased, whereas that distributed in the organelles decreased. The Cd content in the roots and stems of P. hydropiper significantly decreased with the increase in soil pH in the moderate Cd-contaminated soil. In addition, with higher soil pH values, the proportion of Cd distributed in the cell wall decreased, whereas that distributed in the organelles increased. The translocation factor (TF) of P. hydropiper was higher than one in all treatments, indicating that it can effectively transport root-absorbed Cd to the aboveground shoots. Based on the relatively high bioconcentration factor and TF, P. hydropiper has the potential to remediate Cd-polluted paddy soil. Furthermore, the remediation efficiency of P. hydropiper can be enhanced by adjusting the pH as per the soil-Cd pollution.

The heavy metals (Zn, Pb and Zn) concentrations in soil and medicinal plant samples from Aba city, Nigeria using atomic absorption spectrophotometer UNICAM 919 model was evaluated in a randomized complete block design. The results showed that there was a clear accumulation of metals in soil and medicinal plants in relation to vehicular emission in the city. A highest concentration of Zn (133.56±7.70 mg/kg) in soil was obtained from X2, Pb (29.71±1.56 mg/kg) and Cd (21.11±1.28 mg/kg) was in X5. The levels of Zn, Pb and Cd in soil were 15.85±18.03 - 133.56±7.70 mg/kg, 10.06±0.36 - 29.71±1.56 mg/kg and 1.07±0.08 - 21.11±1.28 mg/kg, respectively. In medicinal plants, the highest concentration of Zn was obtained in Azadiractha indica (34.58±2.07 mg/kg), Pb was in Psidium guajava (10.47±0.93 mg/kg) and levels of Zn, Pb and Cd were 9.02±1.40 - 34.58±2.07 mg/kg, 1.55±0.35 - 10.47±0.93 mg/kg and 0.02±0.0 - 1.44±0.11 mg/kg, respectively. The concentrations of metals in soil and medicinal plant were in decreasing order: Zn>Pb>Cd. Zinc in soil significantly correlated positively with Zn (0.956, p<0.01) and Cd (0.631, p<0.01) in plants; Pb in soil significantly correlated positively with Zn (0.825, p<0.01), Pb (0.810, p<0.01), Cd (0.578, p<0.05) in plants. The level of Cd in soil reflected significant pollution compared to concentrations in soils of urban cities in developing countries.

Wheat tends to accumulate higher levels of cadmium in the grains than rice under a wide range of soil pH and Cd concentrations: A field study on rice-wheat rotation farmland.

This study determined the concentration of three heavy metals zinc (Zn), lead (Pb), and cadmium (Cd) in soil and in a woody plant species, Milicia excelsa, at Ishiagu quarry, Nigeria. The highest soil concentrations of Zn, Pb, and Cd in soil were obtained at 1 m from the quarry site. In M. excelsa, the highest concentrations of Zn, Pb, and Cd were 3.12–9.1, 3.9–6.01, and 0.51–1.12 mg kg−1, respectively. There were significant positive correlations between Cd and Zn (r = 0.963) and Cd and Pb (r = 0.974) in plants as well as between Cd and Pb (r = 9.84) in soil. The level of Cd in soil reflected significant pollution compared to average global concentrations in soils.

This study assessed the distribution of heavy metals in soil and their subsequent accumulation in plants at a site at Umudike, Nigeria, that had been contaminated by agrochemicals. Soil and plant samples were analysed for zinc (Zn), chromium (Cr) and cadmium (Cd). The highest concentrations of Zn (251.50 mg/kg) and Cd (61.33 mg/kg) were obtained at a soil depth of 0–10 cm. The highest concentrations of Zn (16.52 mg/kg), Cd (27.12 mg/kg) and Cr (164.07 mg/kg) were accumulated by Baphia nitida. The levels of Cd, Cr and Zn in soil were 27.97–61.33, 24.97–45.43 and 148.57–251.50 mg/kg, and their concentrations in B. nitida were 16.18–27.13, 97.99–164.07 and 0.10–16.52 mg/kg, respectively. There were significant correlations between Cd and Cr and Cd and Zn in soil, as well as between Cd and Cr in plants. The concentration of Cd in soil reflected a state of pollution relative to Dutch criteria for soil and the FAO/WHO Codex Alimentarius Commission.

Adsorption and precipitation of cadmium (Cd) could be dependent on rate of P addition and Cd level in soil. Therefore, the objective of this study was to examine how addition rate of P affect mechanisms of Cd immobilization such as adsorption and precipitation in different levels of Cd in soil. Arable soils were spiked with inorganic Cd (CdCl₂) to give a total Cd concentration of 10, 100, and 1,000 mg Cd kg -1 . Monopotassium phosphate (KH₂PO₄, MPP) was selected as phosphate material and mixed with the pretreated arable soil at the rates of 0, 800, 1,600 and 3,200 mg P kg-1. The mixture soils were incubated at 25°C for 8 weeks in dark condition. Soil pH decreased with increasing MPP addition rate in all levels of Cd but negative charge of soil increased, thereby reducing 1 M NH₄OAc extractable Cd. Soil solutions were undersaturated with respect to CdCO₃ and Cd₃(PO₄)₂ with all P addition rate in soil with low Cd level (≤ 100 mg Cd kg-1) but supersaturated in soil with high Cd level (1,000 mg Cd kg-1). From the above results, Cd solubility was controlled by precipitation of Cd minerals such as CdCO₃ and Cd₃(PO₄)₂ in soil with high Cd level but by Cd adsorption induced by increase in negative charge of soil with low level of Cd.

Phytoremediation of heavy metal-contaminated soil is considered to be one ecological environmental protection way that is effective and economical. The selection of suitable hyperaccumulators is a key issue for phytoremediation of heavy metal-contaminated soil. Pot experiments were conducted to study the effects of different Cd levels (0, 75, 150, 300, and 600mgkg-1 Cd) on the dry weight, antioxidant enzyme activities, malondialdehyde (MDA) contents, Cd concentration, Cd accumulation, and soil Cd form distribution ratio (FDC) of alfalfa (Medicago sativa L.) and Indian mustard (Brassica juncea L.). The correlations between Cd concentration in shoots and roots of alfalfa and Indian mustard and soil Cd form were also investigated. The results showed that with the increase of soil Cd levels, dry weight of shoot and root of alfalfa and Indian mustard significantly decreased, which decreased by 50.0-71.8% and 29.6-59.3% (alfalfa), 59.6-89.0% and 64.3-74.8% (India mustard), respectively, compared with the control. With the increase of soil Cd level, superoxide dismutase (SOD) activity in shoot of alfalfa significantly increased. Catalase (CAT) activity and malondialdehyde contents in shoots and roots of alfalfa and Indian mustard, as well as superoxide dismutase activity in the roots of alfalfa and Indian mustard increased first and then decreased with the increase of soil Cd level. With increasing Cd stress, Cd concentration in shoots and roots of alfalfa and Indian mustard significantly increased. At soil Cd level of 75mgkg-1, Cd concentrations in shoots of alfalfa and Indian mustard exceeded the critical value of Cd-hyperaccumulator (100mgkg-1), which was 356.46mgkg-1 and 308.74mgkg-1, respectively. Cadmium concentrations in shoots and roots of plants were in the order of that of alfalfa > Indian mustard; total Cd accumulation in the aboveground tissues and roots of the plants was in the order of that of Indian mustard > alfalfa at the same Cd level. With increasing soil Cd level, Cd concentrations of exchangeable form (EXC-Cd), carbonate-bound form (CAB-Cd), iron-manganese oxide-bound form (FeMn-Cd), organic-bound form (OM-Cd), and residual form (RES-Cd) showed an increasing trend. The form distribution ratio of soil Cd forms in alfalfa's rhizosphere was in the order of that of exchangeable form Cd > carbonate-bound form Cd > iron-manganese oxide-bound form Cd > residual form Cd > organic-bound form Cd. Except for organic-bound form Cd, soil Cd forms were significantly positively correlated with Cd concentration in shoot and root (P < 0.01). Comprehensively considering the biomass and Cd accumulation, Indian mustard is more suitable as remediation material for soil Cd pollution.

A pot experiment was conducted in a greenhouse to establish the threshold level of Cd in a light soil planted with cereals. During three consecutive years barley, wheat, rye, and corn were grown in the soil treated with 2.5; 5; 25 Μg g−1 of Cd applied in sulphate form. The Cd concentration of 5 Μg g−1 (plus native Cd content of the soil) was considered as the maximum permissible Cd level in a soil for the soil-plant systems studied. Besides the estimation of total Cd concentrations, several forms of Cd in soils (exchangeable, carbonate, Fe-Mn oxides, organic, and residual) were determined by mean of sequential extraction to define the phytoavailable form of Cd in the soil. The Cd concentration in studied plants increased with the doses of this metal in soil but to different extents for various plant species and various plant parts. The highest Cd enrichment ratios (ER) were found for wheat and corn leaves (2 to 3), whereas, the lowest ERs were detected for corn and rye grains (0.05 to 0.2) grown at the treated soils. All samples of the control plants had ER below 1. A close relationship was found between Cd content of plants and the exchangeable form of Cd in soil, which indicates that this form of the element is readily available for plants. The proposed maximum permissible level of Cd (5 Μg g−1 of added metal plus native Cd content of soils) in light soils proved to be too high for cereal plants. The threshold concentration for light soil should not exceed 3 Μg g−1.

In a pot experiment, three N-fertilizers, differing in NO−3 and NH+4; content, were compared in terms of their effects on the extractability of soil Cd in 1M ammonium acetate at pH 7 and on the uptake of native and added Cd by winter rape (Brassica napus L. var. oleifera Metzger). In another similar experiment the Cd-availability in soils receiving NPK-fertilizer applied in a concentrated, granulated form was compared to that in soils receiving a uniform application of dissolved NPK-fertilizer. Both loamy sand and clay soils were used. With the N-fertilizers levels of extractable soil Cd and plant Cd-contents were lowest in the nitrate of lime treatment, highest in the ammonium sulphate treatment and intermediate in the nitro-chalk treatment. The addition of 1 mg kg−1 DW Cd to the soil increased Cd-levels but did not change the response pattern. There was a strong association between the effects of the various fertilizers on Cd availability and their effects on soil pH: the stronger the acidifying effect the higher the Cd availability. When applied at a low dose, granular NPK was more effective than dissolved NPK at enhancing plant uptake of Cd from both soils, but this was not the case when applied at a higher dose to the sand. The two forms of NPK differed little in their effect on extractable soil Cd. Plant uptake of Cd was greater from soils receiving granular NPK with a high Cd-content than from those receiving granular NPK low in Cd. In both experiments extractable Cd was taken up to a greater extent from the sand than from the clay. An increase in fertilizer dose generally resulted in an increase in levels of soil-Cd and in Cd concentrations in the plants.

Magnetic biochar-supported nanoscale zero-valent iron for remediation of arsenic and cadmium-contaminated soils: The role of free radicals.

To investigate the levels of heavy metal Cd in paddy soils in high cadmium anomaly areas in Guangxi, and to assess their potential ecological risks to the environment. Through preliminary screening and detailed investigation, 912 pieces of soil samples from high-cadmium abnormal area were collected in multiple batches to determine the soil Cd content. The single-factor pollution index method and potential risk index method were used to control the Cd pollution degree of paddy soil. Potential risks were evaluated. The results showed that:① The average Cd values of natural soil, paddy soil, and dryland soil in the initial screening were 0.915, 0.591, and 0.593 mg·kg-1, respectively. ② In the detailed investigation, the soil pH is 4.6-8.7, which is between acidic and weakly alkaline. If the Soil Environmental Quality Standard (GB 15618-2018) is used as the evaluation standard, the Cd of the soil sample in Pingguo County, Tiandong County, Long'an County, and Liucheng County is seriously exceeded, and the soil sample in the paddy field of Rongshui Country is not polluted. Based on the soil baseline value, the Cd in the soil samples of Tiandong County, Liucheng County, and Rongshui County were non-polluting. In the paddy soils of Tianyang County, Pingguo County, Tianxian County, Daxin County, Long'an County, and Rong'an County, the proportion of Cd in mild to moderate pollution was 4.2%, 3.7%, 14.9%, 2.6%, 7.1%, and 1.4%, respectively. ③ Cd in paddy soils of nine counties and cities presents different levels of potential ecological risks. The soil Cd of some paddy fields in Tiandeng County, Daxin County, and Long'an County was at a high ecological risk ratio of 4.3%, 2.6%, and 2.4%. The soil Cd of Tianyang County, Pingguo County, Rong'an County, and Liucheng County was medium-Middle and high potential risks. Tiandong County and Rongshui County are at low potential ecological risks. In conclusion, the overall Cd in the paddy soil of the study area is high, which may affect the safe planting of rice in the long-term. It will eventually pose a health threat to local residents and should be taken care of. It is recommended to carry out research on soil cadmium bioavailability and rice cadmium accumulation in the study area in order to assess its ecological risk and health risk more scientifically and reasonably.

A greenhouse experiment examined lead (Pb) and cadmium (Cd) content of white mustard (Sinapis alba L.) on soils of varying Pb and Cd content. Plants were grown for 29 days. Fresh and dry plant biomass (root, stem, leaves) was measured. Cadmium and Pb concentrations of separate plant parts were determined by atomic absorption spectrometry. Total and available Cd and Pb contents of the soils were determined. Positive correlations were found between Cd and Pb contents of soils and plant uptake. Concentrations of Cd increased in an measured plant portions with increasing levels of available soil Cd. As available Pb increased in the soil, Pb concentrations increased in the root only. The highest concentration of Pb was measured in the roots. The highest levels of Cd were measured in the leaves.