The use of NAA in this study was limited by its reliance on delayed detection, which, while reducing immediate dead-time effects, significantly increased the time required for sample analysis. This extended counting time hindered analytical throughput and efficiency. To overcome these limitations and enable faster, more sensitive quantification of heavy metals, inductively coupled plasma mass spectrometry (ICP-MS) was employed as a complementary technique. Soil samples were digested using a wet acid digestion method with a nitric acid to perchloric acid ratio of 3:19,13. The resulting solutions were analyzed using a Perkin Elmer ELAN 9000 ICP-MS, providing rapid, multi-element detection with high sensitivity and low detection limits.
Figure S1 (please refer supplementary) illuminates the XRD spectra of the Condisoil, in which the presence of other peaks corresponding to the gypsum compound according to the crystallography Open Data (COD) card No. 96-101-1075 was clearly visible aside from the soil conditioner. Thus, the presence of gypsum indicated by XRD analysis shows that Condisoil is suitable for use in agriculture which is consistent with earlier findings by Hanafi et al..
The application of Condisoil a had notable effect on the chemical and physical properties of soils across the three study locations: Alor Pudak (AP), Merbok (MB), and Penaga (PN). One of the most significant outcomes was the marked increase in available phosphorus (P) concentration in all soils post-treatment. In Alor Pudak, available P more than doubled (from 1.29 ± 0.3 to 2.99 ± 0.29 mg/kg), while in Merbok, it increased more than fourfold (from 0.77 ± 0.19 to 3.20 ± 1.09 mg/kg). Penaga showed a similar trend, with available P tripling (from 0.95 to 2.92 mg/kg). These results suggest that Condisoil effectively enhanced P availability, likely through improved nutrient solubility, reduced fixation by Fe/Al oxides, or increased microbial activity that promotes P mineralization.
Interestingly, this increase in phosphorus occurred despite relatively minor changes in soil pH. The pH remained in the acidic range (between 3.69 and 4.12) across all locations, with only slight fluctuations observed after treatment. This implies that the improved P availability may not be solely due to pH modification, but rather the Condisoil may have provided buffering effects or included organic matter that facilitated P release or mobility. In terms of exchangeable cations, the effects were more variable. Exchangeable potassium (K) generally increased or remained stable post-treatment. The most notable improvement was seen in Merbok, where K nearly doubled (0.56-1.09 cmol⁺/kg), potentially alleviating K deficiencies and enhancing nutrient balance. Conversely, exchangeable calcium (Ca) and magnesium (Mg) showed slight decreases in Alor Pudak and Merbok but increased in Penaga. These fluctuations may indicate cation displacement or redistribution as part of the nutrient release mechanism of the Condisoil. The increases observed in Penaga suggest that the Condisoil may contribute to cation enrichment under certain soil conditions.
Changes in soil texture components (sand, silt, clay) were also observed, with slight increases in finer particles (silt and clay) in most cases. This could be attributed to improved soil aggregation and the binding of finer particles facilitated by the organic or humic components of the Condisoil. Enhanced soil structure would promote better water retention and nutrient-holding capacity, indirectly contributing to greater nutrient availability. Overall, the soil conditioner demonstrated a positive effect on soil fertility by significantly increasing phosphorus availability and improving cation balance, particularly potassium. These changes, combined with improved physical characteristics, suggest that the conditioner has strong potential as a soil amendment in acidic tropical soils with limited nutrient availability.
Based on Table 2, Condisoil shows the same potential to improve soil properties as other soil conditioners derived from industrial by-product. The use of different soil conditioners depends on the specific soil properties of the studied location. Some soil types require amendments to increase pH levels, while others benefit more from enhanced phosphorus (P) availability, as demonstrated in the work by da Costa Leite et al., where a mixture of 25% bauxite residue (BR) and 75% palm oil compost (POC) significantly increased P availability. This improvement led to a growth response in Brachiaria grass, highlighting the importance of tailored soil amendments based on specific nutrient limitations and crop requirements. Such findings reinforce the role of organic-industrial by-product blends in sustainable soil management practices. Therefore, selecting the appropriate soil conditioner is crucial and should be based on targeted soil limitations to optimize crop productivity and soil health.
On its own, Condisoil was seen to show a slight elevation of NOR concentration compared to the average soil concentration in Malaysia as shown in Table 3. However, the for Ra and Ra concentration in Condisoil were below the limit value of 1000 Bq kg approved by the authority which were 123.0 ± 6.1 Bq/kg and 120.8 ± 3.1 Bq/kg respectively. This, as mentioned prior, is attributed to that it consists of 10% radioactive waste, specifically the WLP residue.
The activity concentration ranges before applying Condisoil for Ra, Ra and K in the first season were 55.6-150.9, 44.8-74.3 and 374.8-481.9 Bqkg while for the second season when applied as a soil conditioner, they were 60.6-155.5, 41.9-76.1 and 357.9-517.0 Bqkg, respectively. The results indicated that all soil samples treatment with Condisoil possesses an activity concentration slightly higher than the average soil for Malaysia but still within the paddy cultivation soil range previously reported (refer Table 3).
Statistical analysis via the ANOVA test shows that the use of Condisoil for the first seasons (control condition without Condisoil) and second seasons (applying Condisoil) of paddy cultivation indicated no increase in NOR concentration in the environment, as the p-values were > 0.05, leading the study to conclude that the application of Condisoil did not result in an increase in natural radionuclides in the soil over the study period.
Table 4 showed that the average concentration of NOR in rice grains sample treated with Condisoil were seen to be in the activity concentrations range of 0.93-9.20 Bq/kg, 0.03-0.53 Bq/kg and 89.14-147.95 Bq/kg for Ra, Ra and K respectively. Comparing said values with the values obtained in this study, it was revealed that the rice grains from the treated soil were within the range of other countries’ paddy fields reported previously. To better contextualize the statistical significance of the results in terms of potential environmental impact, a correlation analysis was performed to assess the relationship between the activity concentrations of NOR (Ra, Ra, and K) in rice grains and those in the surrounding paddy soils from Season 2, following the application of a soil conditioner.
As presented in Fig. S2, the analysis revealed strong positive correlations for Ra and K in samples from Penaga and Alor Pudak, with R values approaching 1.00. These statistically significant correlations suggest that the elevated levels of Ra and K in rice grains are likely influenced by their respective concentrations in the soil at these locations. This implies a direct pathway for the transfer of these radionuclides from soil to crop, thereby raising concerns about potential environmental and human health impacts due to long-term consumption of contaminated rice.
Notably, the soil at Penaga and Alor Pudak exhibited the highest concentrations of Ra and K, respectively, further supporting the likelihood of bioaccumulation in rice grains. In contrast, other locations showed lower correlation values, particularly for K, indicating spatial variability and less consistent soil-to-grain transfer dynamics. The detailed discussion on potential NOR bioaccumulation will be further evaluated through transfer factor (TF) analysis in Section “Transfer factor (TF)”. These statistically significant findings emphasize the importance of continued monitoring and risk assessment, especially in areas where elevated radionuclide concentrations in soil coincide with food production. A more detailed evaluation of the potential radiological risk associated with rice grain consumption is presented in Section “Radiological impact assessment of consumption rice grain samples due natural radionuclide after applying Condisoil”, which quantifies these risks using established radiological health metrics.
A) Daily intake of radionuclide D.
The daily radionuclide intake (D) was determined by the radionuclides content in rice grains and their accumulation in the human body as a result of ingestion by an average adult. D was calculated using the following Eqs S1 (please refer Table S2 in supplementary material), taking the annual per capita intake rate for Malaysia at 82.3 kg/year into account:
The estimated daily intakes of radionuclides D via the consumption of the treated rice were within the range of 0.24 — 2.39 Bq/kg, 0.02 - 0.08 Bq/kg and 23.15 - 38.43 Bq/kg for Ra, Ra and K, respectively.
b) Annual effective dose.
Annual effective dose (AEDE) estimations of internal exposure via ingestion of Ra, Ra, and K in rice grains samples were done towards different age groups (1 yr, 5 yr, 10 yr, 15 yr, and adult >17 yr) of the community. The AEDE in mSv/y was estimated using the activity concentrations of the radionuclides according to Eqs S2 which can be found in Table S2 in supplementary material :
Findings indicate that Ra contributed the highest exposure of 0.02 – 0.58 mSv/y in this study compared to Ra and K as shown in Table 5. This could be attributed to the fact that Ra is more soluble in soil than Th series nuclide belongs to Ra, thus resulting in a higher affinity for the soil’s daily exchange sites. This would allow rice plants to rapidly absorb radium from the soil along with other minerals required for growth. In addition, the average AEDE levels of rice from Condisoil-treated-locations were in the range with previous research (refer Table 5).
On average, the intake of NOR in their food is approximately 0.3 mSv/y. On the other hand, AEDE levels in rice grain treated with Condisoil were slightly higher than 0.3 mSv/y for the youth of community. However, the cumulative AEDE for all age groups were below the ICRP reference limit of 1.0 mSv/y.
c) Annual organ-specific equivalent dose (AOED).
Figure 2 summarises the estimation findings of the annual organ-specific equivalent dose (AOED) caused by the ingestion of rice grains in the sample region for adults using the Rad Toolbox. The calculations of average radionuclide activity concentration in rice of the Alor Pudak study area and the intake rate were also included in this study by using Eqs S3 (please refer Table S2 in supplementary material).
Annual organ equivalent doses for K, Ra, and Ra in rice grain ranged between 0.05-0.20 mSv/y with an average of 0.07 mSv/y, 0.01-4.11 mSv/y with an average of 0.34 mSv/y, and 0.01-0.60 mSv/y with an average of 0.05 mSv/y, respectively, as illustrated in Fig. 2. Using the Rad Toolbox, the illustration shows that the equivalent doses of K, and Ra were less than the average AEDE population worldwide value of 0.3 mSv/y for different parts of the body.
However, for Ra, exposure to bone surface could potentially exceed 0.3 mSv/, ranging between of 0.6-4.2 mSv/y. This is likely due to the fact that radium is treated as calcium in the human body, thus having higher propensity to accumulate in the bones which results in an elevated exposure dose in the bone surface region when radium intake is high. This study believes that Condisoil-tread rice grain should be subject to a milling process process that may reduce impurities that can further minimize body exposure.
Apart from NOR, heavy metal concentrations in soil and rice grains were also calculated and summarised in Table 6. Five heavy metal elements were identified in this study: As, Sb, Cr, Cr and Pb. In comparison to the NOR content, the heavy metal content in Condisoil were found to be lower than the Malaysian soil standard value (refer Table 6).
In Condisoil-treated-soil on the other hand, heavy metal concentrations were found to be less than the Malaysian soil standard value and within the range found in the previous study (refer Table 6). Additionally, the concentration of heavy metals in rice grain is lower than previous study (refer Table 6). Though that may be the case however, it is critical to quantify the risk posed to individuals who ingest said rice grains, which will be discussed in detail in Subtopic 3.3.2.
The following Eqs S4 was used to determine the average daily intake (EDI) of heavy metals from rice consumption after applying Condisoil. Daily intake (EDI) levels of As, Sb, Cr, Cd and Pb metals via Condisoil-treated-rice consumption were within the range of 1.24 × 10-1.66 × 10, 8.29 × 10-5.80 × 10, 4.15 × 10-1.24 × 10, 4.15 × 10-8.29 × 10 and 2.65 × 10-9.00 × 10 mg/day kg BW, respectively, as shown in Table 7.
The THQ is used to quantify the potential health effects of long-term rice consumption. It represents the ratio of the EDI to the reference oral dose (RfD) for each heavy metal, as determined by Eqs S5. THQ values for As, Sb, Cr, Cd, and Pb were 0.41-0.55, 0.21-1.45, 0.00-0.01, 0.41-0.83, and 7.58-21.44, respectively, for all Condisoil-treated soils. As, Cr, and Cd showed THQ values below one, indicating no potential adverse health effects. With the exception of Sb, the THQ value at Alor Pudak was somewhat higher than one, posing a health risk to the public. This study suggests that the slightly elevated Sb levels in Alor Pudak rice may be linked to pesticide usage. Since the rice sample (unpolished) in this study did not undergo the previously mentioned rice milling process, residual pesticides may not have been fully removed during sample preparation. If the rice were to undergo this milling process, the THQ value for Sb is expected to fall below one. Wu et al. (2024) reported that cadmium (Cd) concentrations were significantly lower in polished grains compared to unpolished grains. Consequently, their study incorporated parallel analyses of both grain types to enhance the robustness and resolution of the dataset. For future large-scale monitoring, it is imperative to conduct comparative analyses of polished and unpolished grains to capture the full extent of heavy metal distribution and potential human exposure.
In addition, the THQ value for Pb exceeded one across all three studied locations, potentially due to the cultivation of rice in flooded paddy fields, which enhances the mobility and bioavailability of heavy metals such as Pb. Furthermore, contamination of soil and irrigation water, particularly with pesticides and phosphate-based fertilizers, may contribute to elevated Pb levels in the agricultural environment. As a result, rice plants can uptake and translocate Pb into edible grains, thereby increasing dietary exposure among consumers.
When more than two contaminants are present, cumulative effects might occur. Hence to assess the presence of said effects, the hazard index (HI) was used to analyze the potential adverse health risks as a consequence of heavy metals exposure in rice as described by USEPA document. The following Eqs S6 is used to determine this parameter, which is defined as the total of THQs.
The HI value for all three studied locations was greater than one (refer Table 7), which could be attributed to the presence of Sb and Pb elements from the use of pesticides, fertilizers, and other factors previously discussed. However, the THQ values for Sb and Pb may be reduced if the rice grain samples in this study undergo the rice milling process during sample preparation, and the HI will decrease accordingly. Limitation of this study is that it does not explore the effects of pesticide and phosphate-based fertilizer usage on rice grains, as these factors were not within the scope of the research objectives. Nevertheless, this study suggests that the impact of heavy metal intake from rice should be closely monitored.
A vital component in assessing the impact towards both the environmental and end consumer is to study the transfer factors (TFs) as early preparations and treatments of any agricultural product ultimately affect the end products. TFs is mathematically defined as the ratio of the radionuclide of interest concentration in plant parts (Bq/kg, dry weight) to the elements of interest concentration in the soil.
Table 8 summarizes the calculated TFs of the radionuclides and heavy metals. In this investigation, we observed that the radionuclide, As, Cr and Pb heavy metal transfer factor value reported in prior studies was higher than the transfer factor discovered in this study. However, the TF value for cadmium (Cd) and antimony (Sb) in this study offer critical insights into the uptake potential of these toxic elements in rice cultivated in Condisoil-treated soils. For cadmium (Cd), TF values reached 1.0 at both Alor Pudak and Merbok, while a lower value of 0.5 was recorded at Penaga. These values align with transfer factors reported in contaminated agricultural systems, such as 1.17 in fly-ash amended soils and 1.2 in a mining-affected paddy region in China. The high TF value in this study, particularly at Alor Pudak and Merbok, suggest efficient translocation of Cd from soil to rice grains, raising potential concerns regarding food safety. Cadmium is a known nephrotoxicant and carcinogen, and even moderate levels of dietary exposure have been associated with adverse health outcomes. Therefore, its bioavailability in paddy fields treated with Condisoil necessitates close monitoring.
The transfer of antimony (Sb) exhibited greater variability across locations. The highest TF was recorded at Alor Pudak (0.60), followed by Merbok (0.30) and Penaga (0.11). Compared to the limited literature, such as a TF of 0.26 observed in Hunan Province paddy fields, the Sb uptake at Alor Pudak is notably elevated. This may be attributed to localized contamination, historical agrochemical usage, or potential interactions between soil characteristics and Condisoil components that enhance Sb solubility and mobility. Although Sb is less studied in agricultural systems compared to Cd or Pb, its classification as a potentially toxic element underscores the importance of its inclusion in risk assessments, especially when detected at elevated levels in staple crops like rice.
In summary, Sb demonstrated notable transfer from soil to rice grains in Condisoil-amended fields while Cd have high potential active uptake and bioaccumulation in plant. These findings highlight the importance of site-specific risk evaluation and the incorporation of heavy metal monitoring protocols in sustainable agriculture practices, particularly when using industrial by-products as soil conditioners. Ensuring food safety while leveraging soil amendments requires a balanced, evidence-based approach that considers long-term implications for public health and food security. The results of this study indicate that rice plants grown in paddy fields treated with Condisoil have a low capacity for uptake and translocation of radionuclides. However, once Condisoil is commercialized, its environmental impact should be closely monitored especially Cd and Sb heavy metal from the soil through the roots system to the grains.
As more and more rare-earth metals are produced daily, the quantity of radioactive residue would follow suit, growing as the years go by as concerns and negative perceptions amongst locals began to aggrandize over time. While the national economy benefits from the labour of the industry, it is in the local community’s best interest to ensure the public continues to accept and embrace the rare-earth processing technology: any considerable reduction in public acceptability of the technology may have a detrimental impact on the government’s policies. At current stage Malaysia government urgency for Malaysia to find a solution for the massive volume of VLLW radioactive waste management. In light of increasing public concern regarding the environmental and health impacts of radioactive waste from rare-earth (RE) processing, the Malaysian government has introduced stricter regulatory conditions for industry operations. A key requirement for future licence applications is the mandatory separation or extraction of thorium to prevent the generation of radioactive waste at processing facilities. Pervious works conducted on the electrosorption-based remediation of thorium from WLP residue has indicated a reduction of approximately 40% in the waste residue volume. The concept of reusing WLP residue is to separate thorium from WLP for thorium potential application as a nuclear fuel in Generation-IV nuclear reactors. Besides, the lack of global deployment of a thorium reactor, coupled with the restricted separation and purification of thorium to laboratory-scale operations, has necessitated the alternative ways like Condisoil product to solve the massive radioactive volume. Therefore, it is imperative to identify the best option to either treat or repurpose the residues produced while concurrently expanding the Malaysian RE business.
Each country planning to reuse this industrial by-product must establish standardized operating procedures (SOPs) specific to rare earth (RE) activities, as this Condisoil was generated from byproduct RE industries and is regulated to ensure the safety of both environmental and human health. For example, the United States Environmental Protection Agency (US EPA) establishes detailed regulations under the Part 503 Rule, which defines acceptable limits for heavy metals in biosolids utilized in land application. In a similar manner, the European Union has set forth standards under Directive 86/278/EEC regarding environmental protection in the agricultural use of sewage sludge, which encompasses maximum permissible concentrations for heavy metals in both the soil and the sludge. The findings of the study suggest that the application of Condisoil does not contribute to elevated levels of natural radionuclides. However, there is a minor potential for the introduction of heavy metals into the environment. Although the radiological impact on human health appears negligible, ongoing toxicological monitoring is recommended over extended periods to comprehensively assess long-term exposure risks. As mentioned before, NUF and WLP’s gypsum properties in Condisoil can be used as an alternative to conventional fertilizers to supply Ca, Mg and/or S, soil conditioner and/or liming agent in ameliorating deficiencies resulting from soil acidity conditions. Condisoil is currently applied to paddy crops with acidic soil issues. If Condisoil were developed and widely used as an alternative to conventional fertilizers as it is anticipated to deliver nutrients to plants, more rare earth process residue would be utilized to manufacture Condisoil, which could mutually benefit both the agricultural and the REE industries. Thus, the plausible conclusion that could be drawn from this work paves the way for a possible alternative for lowering the volume of processed rare-earth residue especially radioactive residue WLP if Condisoil is utilized as an alternative fertilizer.
Around 900 kg Condisoil containing 10% WLP residue was used to grow one paddy hectare. According to Eq. (1), approximately 90 kg of WLP is utilized to manufacture 900 kg of soil conditioner Condisoil.
where mass of residue WLP in Condisoil (kg) per one hectare paddy cultivation, is residue percentage in soil conditioner Condisoil which in this study is 10%, and is mass of Condisoil for one hectare paddy cultivation (kg).
Table 9 shows the annual cultivation area paddy field in Malaysia with the state of Kedah having the largest rice plantation, roughly 215,065 hectares. According to Eq. (2), approximately 193.6 × 10 tonnes of Condisoil will be required to cultivate the entire rice plantation areas in Kedah. The application of 193.6 tonnes of Condisoil results in approximately 19,356 tonnes of WLP residue, as determined by Eq. (3).
where mass required by Condisoil for paddy cultivation (tonnes), is area of paddy cultivation were tested (hectar) and mass of Condisoil for one hectare of paddy cultivation (tonnes).
where is mass of WLP residue containing in Condisoil for paddy cultivation (tonnes), is area of paddy cultivation were tested (hectare) and is mass of residue WLP in Condisoil per one hectare paddy cultivation (tonnes).
Table 9 shows how much in tonnes the required volume of WLP to be employed in every state in Malaysia if Condisoil is marketed for local agricultural activities for paddy only. If this material is to be applied to all paddy cultivation in Malaysia, its grand total of approximately 619,132 hectares, requiring roughly 55,513 tonnes of WLP to be utilized in the agricultural sector every year. Statistically, this would result in a 74% usage of the annual residue production (75,000 tonnes), which by extension, reduces the annual volume of residue needing to be storage by said amount as estimated by Eq. (4):
where waste reduction (%) is a reduction percentage of WLP when applied in agriculture purposes per year, is the summation of mass of WLP for every state in Malaysia per year (in tonnes) and 75,000 is the annual mass of WLP produced from rare earth processing activities in Malaysia.
This estimation is of course done based on the experimental period of one year and all environmental and individual exposure assessments were done based of the same findings. However, it could not be denied that the findings produced in this study showed promising outcomes which-if utilized in the long run could be the long-term solution to this ongoing issue. That being said however if this material were applied for more than a year and is intended to be commercialized in the future, this study strongly suggested that the duration of the pilot test should be extended beyond a year and that monitoring of the effects of the use of Condisoil should be done periodically.
Given the potential risks associated with heavy metal accumulation in agricultural systems, this study emphasizes the importance of implementing stringent monitoring protocols and standardized operating procedures (SOPs) for the large-scale application of soil amendments such as Condisoil. Although such amendments may offer benefits for soil remediation or fertility, their use must be accompanied by government-enforced guidelines to ensure that toxic metal levels remain within safe limits. Without adequate oversight, there is a risk that heavy metals may enter the food chain, posing significant health concerns to consumers. Therefore, the establishment of comprehensive regulatory frameworks is essential to mitigate these risks and protect public health.
