Results of the acid leaching method show that calculated TOC and TIC contents of the four samples are anomalous (Table 1). On the one hand, the TC content of Sample #1 is 12.01%, while the carbon content of its decarbonated sample is 12.70%. According to Eqs (5)–(7), the TOC, TIC, and CaCO3 contents determined by the acid leaching method are 0.16%, 11.85%, and 98.76%, respectively. However, results from other two methods on the Sample #1 show significant differences (Table 2). The TOC content retested by the acid fumigation method is 4.95%, while the TIC content is 7.06% converting into the CaCO3 content of 58.81%. Results of the gasometric method show that the TOC, TIC, and CaCO3 contents are 5.2%, 6.8%, and 56.7%, respectively. For Sample #2, the TC content is slightly greater than 12.00% (~12% after rounding to two decimal places), while the TOC, TIC, and CaCO3 contents calculated by the acid leaching method are –0.05%, 12.05%, and 100.42%, respectively. Results from acid fumigation method retest of Sample #2 show that the TOC content is 3.35%, the TIC content is 8.64%, and the CaCO3 content is 72.03%. Results from gasometric method retest of Sample #2 show that the TOC, TIC, and CaCO3 contents are 3.4%, 8.6%, and 71.7%, respectively. For these two samples (Samples #1 and #2), their TC contents are slightly greater than 12%. Meanwhile, their OC′ values measured by the acid leaching method are also slightly greater than 12%. These TC and OC′ values consequently make the calculated TOC contents to be almost zero or even negative and the TIC contents to be ~12%, resulting in CaCO3 contents close to or exceeding 100%. However, the reasonable results are obtained by the acid fumigation method and gasometric method. Their differences of the TOC and TIC contents obtained between the two methods are about 0.2%.
Sample # Sample name TC/% OC′/% TOCL/% TICL/% CaCO3L/% 1 TJ-B16-UP11+12 12.01 12.70 0.16 11.85 98.76 2 TJ-B16-UP13 12.00 12.66 –0.05 12.05 100.42 3 TJ-C16-MD01 11.31 11.59 19.74 –8.43 –70.29 4 TJ-G1602-UP19 11.71 11.93 50.52 –38.81 –323.39 Note: Subscript L is the result of acid leaching method.
Table 1. Test datum and calculation results of the abnormal values when tested using the acid leaching method
Sample # Sample name TOCL/% TICL/% CaCO3L/% TOCF/% TICF/% CaCO3F/% TOCG/% TICG/% CaCO3G/% 1 TJ-B16-UP11+12 0.16 11.85 98.76 4.95 7.06 58.81 5.2 6.8 56.7 2 TJ-B16-UP13 –0.05 12.05 100.42 3.35 8.64 72.03 3.4 8.6 71.7 3 TJ-C16-MD01 19.74 –8.43 –70.29 6.75 4.55 37.94 7.0 4.3 36.0 4 TJ-G1602-UP19 50.52 –38.81 –323.39 6.97 4.75 39.56 7.4 4.3 36.1 Note: Subscript L is the result of acid leaching method; subscript F is the result of acid fumigation method; and subscript G is the result of gasometric method.
Table 2. Comparison of the results of three test methods
On the other hand, Sample #3 has a TC content of 11.31%. The TOC, TIC, and CaCO3 contents calculated by the acid leaching method are 19.74%, –8.43%, and –70.29%, respectively. Results from acid fumigation method retest of Sample #3 show that the TOC, TIC, and CaCO3 contents are 6.75%, 4.55%, and 37.94%, respectively. The TOC, TIC, and CaCO3 contents from gasometric method retests of Sample #3 are 7.0%, 4.3%, and 36.0%, respectively. For Sample #4, the TC content is 11.71%, while the TOC, TIC, and CaCO3 contents calculated by the acid leaching method are 50.52%, –38.81%, and –323.39%, respectively. Results of the acid fumigation method on Sample #4 appear more reasonable with the 6.97% TOC, 4.75% TIC, and 39.56% CaCO3. Results from gasometric method retest of Sample #4 show that the TOC, TIC, and CaCO3 contents are 7.4%, 4.3%, and 36.1%, respectively. These two samples (Samples #3 and #4) have TC contents approaching 12% from the negative direction and OC′ values slightly less than 12%. Consequently, the TOC contents calculated by the acid leaching method are much higher than the TC contents, resulting in negative TIC contents and thus largely negative CaCO3 contents. The results of acid fumigation method and gasometric method show high similarity, which could be considered as actual compositions of these two samples.
Pitfalls of acid leaching method for determining organic and inorganic carbon contents in marine sediments
- Received Date: 2019-12-13
- Accepted Date: 2020-03-02
- Available Online: 2020-12-28
- Publish Date: 2020-08-25
Abstract: Organic and inorganic carbon contents of marine sediments are important to reconstruct marine productivity, global carbon cycle, and climate change. A proper method to separate and determine organic and inorganic carbons is thus of great necessity. Although the best method is still disputable, the acid leaching method is widely used in many laboratories because of its ease-of-use and high accuracy. The results of the elemental analysis of sediment trap samples reveal that organic and inorganic carbon contents cannot be obtained using the acid leaching method, causing an infinitely amplified error when the carbon content of the decarbonated sample is 12%±1% according to a mathematical derivation. Acid fumigation and gasometric methods are used for comparison, which indicates that other methods can avoid this problem in organic carbon analysis. For the first time, this study uncovers the pitfalls of the acid leaching method, which limits the implication in practical laboratory measurement, and recommends alternative solutions of organic/inorganic carbon determination in marine sediments.
|Citation:||Chen Ling, Zhifei Liu, Jingwen Zhang, Yulong Zhao, Yanwei Zhang, Adrian R. Fernandez. Pitfalls of acid leaching method for determining organic and inorganic carbon contents in marine sediments[J]. Acta Oceanologica Sinica, 2020, 39(8): 96-102. doi: 10.1007/s13131-020-1631-z|