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. 2020 Feb 27;10:3595. doi: 10.1038/s41598-020-60339-1

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Response surface and contour plots showing the effect of the extraction temperature (X₁), extraction time (X₂) and oxalic acid concentration (X₃) on the yield of LAHP. (a) Response surface and contour plots of yield as a function of X1 and X2. (b) Response surface and contour plots of yield as a function of X1 and X3. (c) Response surface and contour plots of yield as a function of X2 and X3.