


































Energy and Earth Science 
Vol. 7, No. 2, 2024 

www.scholink.org/ojs/index.php/ees 

ISSN 2578-1359 (Print)   ISSN 2578-1367 (Online) 

 

40 

 

Original Paper 

Study on Removal Methods of Heavy Metals from Marine 

Freshwater Products 

Chen Junhao
1
 & Chen Guiqing

1
 

1
 Jinan University, Guangzhou 510630, China 

 

Received: November 5, 2024    Accepted: November 15, 2024   Online Published: January 7, 2025 

doi:10.22158/ees.v7n2p40                  URL: http://dx.doi.org/10.22158/ees.v7n2p40 

 

Abstract  

Nowadays, Marine rivers are seriously polluted, and heavy metals such as arsenic, cadmium, mercury, 

lead, chromium and copper are absorbed by Marine freshwater organisms, seriously endangering 

human health. After boiling Marine river products such as scallop skirt into a concentrated liquid and 

adding sodium carbonate Na2CO3, most heavy metals will precipitate under strong alkaline conditions 

and be removed by filtration. But it doesn't work on tin Sn. Adding calcium hydroxide Ca(OH)2, the 

heavy metal forms a precipitate under strong alkaline conditions. It’s filtered out. Citric acid or 

calcium citrate are strong chelating agents that can form chelating precipitates with heavy metals and 

be removed by filtration. The combination of the above two is better. Filtration loss can be reduced if 

hydrolysis is performed first! 

Keywords 

Marine science, seafood juice, scallops, alkali, calcium hydroxide, sodium carbonate 

 

1. Introduction 

The world pollution is more and more serious, since China’s reform and opening up 40 years of rapid 

economic development pollution even more serious! Marine rivers are polluted, and heavy metals such 

as arsenic, cadmium, mercury, lead, chromium and copper are absorbed by Marine freshwater 

organisms. Cadmium is easily enriched in crustaceans, kelp, seaweed and nori, which seriously 

endangers human health. 

According to Guangdong-domestic and international reports: Pearl River estuary oyster copper 100 

times over the standard. According to Zhihu.com, the pass rate of sea crabs sampled by the state is only 

20%. According to our detection: Shandong Laizhou scallop skirt cadmium content reached 15-34 

times of the standard. Arsenic levels in laver in Lianyungang, Jiangsu province, were 20 times higher 



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than the legal limit. Lianyungang, a large laver processing business owner unexpectedly did not know 

that laver also test heavy metals. Such a serious problem has not yet attracted enough attention from the 

relevant departments! 

 

2. Materials, Equipment and Methods 

Boil the skirt of Laizhou scallop into concentrated liquid, precipitate and filter. Detection results 

(mg/kg): 1# Cd=1.4, 2# Cd=2.7, 3# Cd=4.4, 4# Cd=7.3 and 5# Cd=17. Both exceed national standards. 

Methods of removing heavy metals are now investigated using 3# and 5# samples (Figures 1 and 2). 

2.1 Devices 

2.1.1 The precipitation method, precipitation-chelation method and chelation-precipitation method use 

a 5L triangular cone bottle and 1500w temperature control furnace. 

2.1.2 Hydrolyzation-chelation-precipitation method requires hydrolysis equipment. 

2.1.2.1 Reflux hydrolysis. With a 5L triangular cone bottle, plus a rubber plug, plug in the condensation 

tube, and connect the tap water to cool the hydrolyzed steam, so as not to burn the hydrolysate dry. It 

was heated in a temperature controlled electric furnace and hydrolyzed at 100
0
C at one atmospheric 

pressure (1kg pressure) for 12 hours. Take care to regulate the temperature to prevent excessive boiling 

and dry the hydrolysate liquid. 

2.1.2.2 Reactor hydrolysis.  

Use 304 stainless steel reactor, volume according to need. With an anchor mixer. Run the mixer for 10 

minutes at regular intervals at 40 RPM. Be careful not to fill the reactor too full, 70% can be. 

Hydrolysis at 2kg pressure, 112-118
0
C for 12 hours. Be careful not to heat too much to avoid a burnt 

taste. 

2.2 Citric Acid Hydrolysis 

The above hydrolyzed raw materials are heated and hydrolyzed with citric acid and appropriate amount 

of water calculated by the following formula. 

2.2.1 The amount of citric acid hydrolysis can be calculated by the “double Chen hydrolysis formula”: 

from Chen Junhao and Chen Guiqing. Study on Crystal Structure and Application of Pearl and shellfish 

[M]; December 2018, Jinan University Press, P.143. 

M Hydrolytic acid=(molecular weight of hydrolytic acid/14.0067) × raw protein content × 16%×CC÷Y 

Citric acid is a ternary acid Y=3. 

M Citric acid=(192.125÷14.0067) × (weight of raw material × protein content of the raw material) × 16% 

× CC ÷ 3 

=Weight of raw material × protein content of raw material × CC × 0.7315=weight of raw material × 

nitrogen content of raw material × 6.25 × CC × 0.7315...... ⑴. 

Add the same amount of water, heat to hydrolysis. M stands for weight. Select CC refer to Table 1. 

2.2.2 Neutralization of food-grade hydrated lime.  



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From the inorganic chemistry textbook. 

M calcium hydroxide=(222.284÷384.251) × M citric acid=0.5785×M citric acid... The essay 

In order to prevent residual calcium hydrogen citrate CaH(C6H5O7) and calcium hydrogen citrate 

CaH4(C6H5O7)2 from being converted into calcium citrate Ca3(C6H5O7)2 precipitation, and to 

completely remove heavy metals, so that the hydrolysate reaches PH12-13, the addition of hydrated 

lime requires an increase of 5% of the original amount. Since the saturated solubility of hydrated lime 

Ca(0H)2 at 20
0
C is 0.16(g/100g), PH12-13 is reached at this time. Excess lime can be precipitated and 

filtered out, and the saturated solubility of lime in the solution of 0.16g/100g will not affect the 

subsequent treatment. 

2.2.3 After 5 hours of precipitation, filter. 

2.2.4 Expected production V (volume). 

From Chen Junhao, Chen Guiqing. Crystal structure and application of pearls and shellfish [M]. 

December 2018, Jinan University Press, P.147. 

Product volume V(mL)=raw material weight (g) × protein content of the raw material %×16%×γ ÷ 

Amino nitrogen of the product (g/mL)...... (3) 

2.2.5 Extraction rate γ: reactor hydrolysis γ=50% ~ 70%, reflux hydrolysis temperature is greatly 

affected by temperature, worse in winter, γ=30% ~ 50%. 

The hydrolysis capacity of citric acid is weaker than that of hydrochloric acid, and it is more suitable 

for hydrolysis of seafood with protein content below 30%. 

 

3. Experimental Methods 

3.1 Precipitation Method 

3.1.1 After adding sodium carbonate Na2CO3 to seafood liquid, most heavy metals will precipitate 

under strong alkaline conditions, such as CaHAsO4∙H2O, CaCrO4, CdCO3, Hg2CO3, PbCO3, ZnCO3, 

NiCO3, CuCO3, and then filter out. But it doesn’t work on tin Sn. 

3.1.2 After adding hydrated lime Ca(OH)2 to seafood liquid, all heavy metals will precipitate under 

strong alkaline conditions, such as: 

Ca5(AsO4)3∙OH, Ca4(OH)2(AsO4)2, Cd(OH)2, Cd(OH)3, Cr(OH)2, Cr(OH)3, Cu(OH)2, Hg2(OH)2, 

Ni(OH)2, CA5 (ASO4)3∙ OH, CA4 (OH)2(AsO4)2, CD (OH)2, Cr(OH)3, Cu(OH)2, HG2 (OH)2, 

Ni(OH)2, Pb(OH)2, Pb(OH)4, Sn(OH)2, Sn(OH)4, Zn(OH)2. Filter out. 

3.2 Precipitation-chelation 

After the removal of heavy metals, the seafood juice is strongly alkaline, not delicious, and cannot 

measure amino acids, and must be adjusted to weak acidity. The use of hydrated lime precipitates, 

which must be regulated with citric acid, forms calcium citrate and heavy metal precipitates, which are 

filtered out. This is precipitation-chelation, not precipitation anymore. 

 



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3.3 Chelation-precipitation Method 

Citric acid is a strong chelating agent, able to chelate heavy metals. Calcium citrate and heavy metal 

precipitates are formed by adding hydrated lime, and heavy metals are removed by filtering and 

discarding residue after a long time precipitation. 

3.4 Hydrolyzation-chelation-precipitation Method 

Protein is a large molecule, filtration loss. First, the protein is hydrolyzed into soluble amino acids with 

citric acid, and the hydrated lime is added to form calcium citrate and heavy metal precipitates, which 

are filtered and removed to reduce losses. 

 

4. Experimental Results 

4.1 Precipitation Method 

Sodium carbonate is not a base, classified as salt, called alkaline substances, alkaline weak. 

4.1.1 Scallop solution 100ml+ 7% sodium carbonate + 18ml water, dissolved by stirring, pH≈9. 

Precipitate for 48 hours and filter. Test results are shown in Figure 3, Cd=0.89. 

4.1.2 Scallop liquid 250ml+ 10% sodium carbonate + 25ml water, stir to dissolve, pH>9. Precipitate for 

72 hours and filter. Test results are shown in Figure 4, Cd=0.71. 

4.1.3 Scallop liquid 250ml+ sodium carbonate 15%+ water 38ml, dissolved by stirring, pH≈10. 

Precipitate for 72 hours and filter. Test results are shown in Figure 5, Cd=0.72. 

* Using more than 9% sodium carbonate does not improve results. All Cd˃0.5, not up to standard. 

4.2 Precipitation-chelation Method 

4.2.1 Scallop liquid 300ml+ 2% hydrated lime, stir to dissolve, PH=12. Precipitate for 24 hours, add 

citric acid to PH=8. Chelate, precipitate for 36 hours, filter. Test results are shown in Figure 6, 

Cd=0.39. 

4.2.2 Scallop liquid 350ml+ hydrated lime 2.5%, stir to dissolve, PH=12. Precipitate for 24 hours, add 

citric acid to PH=8. Chelate, precipitate for 36 hours, filter. Test results are shown in Figure 7, 

Cd=0.34. 

4.3 Chelation-precipitation Method 

4.3.1 Scallop liquid 560ml+ 6% citric acid, stir to dissolve, pH=3. Reaction for 12 hours, add hydrated 

lime to pH=12. Precipitate for 12 hours and filter. The test results are shown in Figure 8, Cd=0.21. 

4.3.2 Scallop liquid 560ml+ 6% citric acid, stir to dissolve, pH=3. Reaction for 12 hours, add hydrated 

lime to pH=12. Precipitate for 48 hours and filter. Test results are shown in FIG. 9, Cd=0.21. 12 hours 

of precipitation is enough. 

4.3.3 Scallop solution 100ml+ citric acid 6%, stir to dissolve, pH=3. Reaction for 12 hours, add 

hydrated lime to pH=12. Precipitate for 14 hours. The filtering methods are as follows: 

4.3.3.1 Filter paper filtering. Test results are shown in FIG. 10, Cd=0.44. 

4.3.3.2 1 micron filter filter. Test are shown in FIG.11, Cd=0.4. 



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4.3.3.3 0.5 micron filter Filter. Test results are shown in FIG. 12, Cd=0.28. 

4.4 Hydrolyzation-Chelation-precipitation Method 

4.4.1 Scallop liquid 2120ml+ citric acid 426.5g+ water 1600ml. Hydrolysis for 12 hours, hydrated lime 

neutralization and other treatment after precipitation for 12 hours, filtration. Test results as shown in 

FIG. 13, Cd<0.02, As1.2, Pb0.0600, Hg<0.05, Cr0.21, Cu<0.05; Precipitate for 48 hours and filter. 

Test results are shown in Figure 14, As 0.60; Precipitate for 60 hours and filter. Test results as shown in 

FIG.15, As 0.48, inorganic arsenic 0.15, Cd 0.0040. 

4.4.2 Scallop liquid 1695ml+ citric acid 337.3g+ water 1600ml. Hydrolysis for 12 hours, hydrated lime 

neutralization and other treatment after precipitation for 24 hours, filtration. Test results as shown in 

FIG. 16, Cd 0.020, inorganic arsenic 0.15. 

4.4.3 Scallop liquid 1680 ml+ citric acid 330g+ water 1600ml. Hydrolysis for 12 hours, hydrated lime 

neutralization and other treatment after precipitation for 3 hours, filtration. Test results as shown in FIG. 

17, Cd0.079, inorganic As was not detected, Pb 0.0370, Hg was not detected, Cr 0.42. Precipitate for 6 

hours and filter. Test results as shown in FIG. 18, Cd 0.078, inorganic As was not detected, Pb was not 

detected, Hg was not detected, Cr0.42. Precipitate for 9 hours and filter. Test results as shown in FIG. 

19, Cd0.077, inorganic As was not detected, Pb 0.0284, Hg was not detected, Cr 0.33. Precipitate for 

12 hours and filter. Test results as shown in FIG. 20, Cd 0.072, inorganic arsenic As 0.041, Pb 0.0373, 

Hg 0.011, Cr 0.43. 3 hours of precipitation is enough. 

4.4.4 Scallop liquid 2100ml+ citric acid 499g+ water 1200ml. Hydrolysis for 12 hours, hydrated lime 

neutralization and other treatment after precipitation for 12 hours, filtration. Test results as shown in 

Figure 21, Cr 830.93μg/L, Cu 116.44μg/L, inorganic arsenic 0.36mg/Kg, Cd0.86μg/L, Hg 0.34μg/L, Pb 

8.93μg/L. 

4.4.5 Experimental laver. Total amino acid 32.5%. Test results as shown in Figure 22, total arsenic 20, 

total mercury 0.010, cadmium 0.77, chromium 0.93, copper 13.5, lead 0.508. 

4.4.6 Laver seasoning liquid. 180g dried laver + 258g citric acid + 1600ml water. CC=7, hydrolysis 12 

hours, hydrated lime neutralization and other treatment after precipitation 12 hours, Test results as 

shown in FIG. 23 shows lead < 0.05, total arsenic 0.38, total mercury < 0.05, cadmium < 0.02, 

chromium < 0.1, copper < 0.5. 

4.4.7 Nori 84g+ Citric acid 159.8g+1680ml water. CC=8. Hydrolysis for 12 hours, hydrated lime 

neutralization and other treatment after precipitation for 12 hours, filtration. Test results as shown in 

Figure 24, Cr 92.16μg/L, Cu 130.92μg/L, inorganic arsenic 0.46mg/Kg, Cd 21μg/L, Hg 0.12μg/L, Pb 

4.34μg/L. 

 

 

 

 



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5. Conclusion 

1) It is not possible to remove heavy metals by simple precipitation method, because it is strongly 

alkaline and must be adjusted to weak acidity to make food, which is no longer a precipitation method. 

2) Precipitation-chelation or chelation-precipitation method is feasible to remove heavy metals, the 

latter effect is better! 

3) Hydrolyzation-chelation-precipitation method is the most effective method to remove heavy metals, 

with less filtration loss, and can directly produce amino acids to make seafood condiments and complex 

seasonings. 

 

The book is illustrated 

 
Table 1. 3# Scallop liquid                               Table 2. 5# Scallop liquid  

 

 

Table 3. 7% of edible alkali, precipitated 48 hours           Table 4. 10% of edible alkali, precipitated 72 hours 

 

 

Table 5. Edible alkali 15%, precipitation 72 hours  Table 6. Hydrated lime 2%+ citric acid, precipitation 36 hours 

 

Table 7 Hydrated lime 2.5%+ citric acid, precipitated for 36hours    Table 8 Citric acid 6%+ hydrated lime, 

precipitated for 12hours 



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Table 9. 6% citric acid + hydrated lime, precipitated for 48hours   Table 10. 6% citric acid + hydrated lime, 

precipitated for 14hours 

 

 

Table 11. Citric acid 6% + hydrated lime, precipitated for 14 hours  Table 12. Citric acid 6% + hydrated lime, 

precipitated for 14 hours,  

 1 filter membrane filtrationr                                  0.5 filter membrane filtrationr   

 

Table 13. Scallop hydrolyzed juice , precipitated for 12hours       Table 14. Scallop hydrolyzed juice , 

precipitated for 48hours 

 

 

Table 15. Scallop hydrolyzed juice, precipitated for 60hours       Table 16. Scallop hydrolyzed juice, 

precipitated for 24hours 



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Table 17. Scallop hydrolyzed juice, precipitated for 3 hours       Table 18. Scallop hydrolyzed juice, 

precipitated for 6 hours 

 

Table 19. Scallop hydrolyzed juice, precipitated for 9hours      Table 20. Scallop hydrolyzed juice, precipitated 

for 12hours 

 

 

Table 21. Scallop hydrolyzed juice, precipitated for 12hours     Table 22. Laver 

 



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Table 23. Laver seasoning solution                            Table 24. New laver hydrolyzed juice 

 

Gly 甘氨酸。Ala 丙氨酸。Val 缬氨酸。Leu 亮氨酸。Ile 异亮氨酸。Phe 苯丙氨酸。Trp 色氨

酸。 

Tyr 酪氨酸。Asp 天冬氨酸。His 组氨酸。Asn 天冬酰胺。Glu 谷氨酸。Lys 赖氨酸。Gln 谷氨

酰胺 

Met 甲硫氨酸。Arg 精氨酸。Ser 丝氨酸。Thr 苏氨酸。Cys 半胱氨酸。Pro 脯氨酸。 

 

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Chen Junhao, & Chen Guiqing. (2017). Production method of fresh scallop juice. Patent No. ZL 2015 1 

0571208.3. Authorization date November 21, 2017. 

Chen Junhao, & Chen Guiqing. (2018). Crystal Structure and application exploration of pearls and 

shellfish. Jinan University Press, December 2018. 

Chen Junhao, & Chen Guiqing. (2019). The Ancient Shells Were Sdudied by X-ray Diffraction and 

Electron probes. Journal of Water Resources and Ocean Science, 8(6), 86. 

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Chen, G. Q., & Chen, J. H. (2018). Production method of astaxanthin prawn juice. Patent No. ZL 2015 

1 0571551.8. Authorization date: January 2, 2018. 

Hu Jing et al. (2017). Response Surface method optimization of removal process of cadmium from 

mushroom by citric acid. Food Science, 38(14). 

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properties of brown rice flour. China Journal of Grain and Oil, 32(8). 

 

 

 

 

https://doi.org/10.11648/j.wros.20190806.11

