How can I place a table (including caption) across two columns of a \twocolum document?

I have inserted a table in a two column document. The table should span across both columns without a page break. Ideally, the table should be fixed at the top of a page with two columns of text below. I almost got what I wanted using \FloatBarrier. The table now ignores the columns and spans across the whole page breadth. However, this didn't work for the caption below, which is fit into the left column. I also noticed that in Overleaf the text gets printed over the table and the table is still fixed to a column (when it gets positioned on a right side column, it stretches over the borders of the page). Is there a way to get the result I want (see picture)?

\documentclass[12pt,a4paper,twoside]{scrartcl}

\usepackage[utf8]{inputenc}

\usepackage[left={2.5cm},right={2.5cm},top={2cm},bottom={2.5cm}]{geometry}

\usepackage{multirow}

\usepackage{tabularx,booktabs}

\usepackage{placeins}

\usepackage{calc}

\usepackage{float}

\usepackage{lipsum}

\begin{document}
\twocolumn
\lipsum[3-5]

\FloatBarrier

\begin{table}[!tbh]

\centering

\begin{tabularx}{\textwidth}{lllll}

%\toprule
\textbf{step} & \textbf{extractant} & \textbf{target phase} & \parbox{3cm}{\bfseries possible mechanism} & \textbf{reference} \\
\parbox[t][][t]{\widthof{digestion}}{PO$$_4$$} & \parbox[t][][t]{\widthof{1\,M NaH2PO4}}{1\,M NaH2PO4, pH\,5, 16\,h, 24\,h + water wash} & \parbox[t][][t]{\widthof{As coprecipitated}}{adsorbed As} & \parbox[t][][t]{3.2cm}{anion exchange of PO4 for AsO4 and AsO3} & \parbox[t][][t]{3.3cm}{\cite{welchFactorsControllingShallow1998}; modified by \cite{keonValidationArsenicSequential2001}}\\

\parbox[t][][t]{\widthof{digestion}}{HCl} & \parbox[t][][t]{\widthof{1\,M NaH2PO4}}{1\,N HCl, 2$$*$$\,1\,h + water wash} & \parbox[t][][t]{\widthof{As coprecipitated}}{As coprecipitated with Mn oxides and amorphous Fe oxides} & \multirow{3}{3.2cm}{proton dissolution; Fe-Cl complexation} & \parbox[t][][t]{3.3cm}{\cite{huerta-diazPyritizationTraceMetals1992}; \cite{cornwellCharacterizationIronSulfide1987}; \cite{wallmannNewProcedureDetermining1993}; \cite{canfieldSulfateReductionDiagenesis1988}} \\

\parbox[t][][t]{\widthof{digestion}}{Microwave digestion} & \parbox[t][][t]{\widthof{1\,M NaH2PO4}}{xxx} & \parbox[t][][t]{\widthof{As coprecipitated}}{xxx} & \parbox[t][][t]{3.2cm}{xxx} & \parbox[t][][t]{3.3cm}{cite{various}} \\

\bottomrule
\end{tabularx}

\caption[Single Extraction: Steps]{Single Extraction Procedure; table from \cite{keonValidationArsenicSequential2001} (modified)}

\label{extraction_steps}

\end{table}

\lipsum[6-8]

\end{document}

• Use table* instead of table. May 19, 2019 at 11:59
• Unrelated, but for molecular formulas you might want to load the mhchem package and use \ce{NaH2PO4}... Also useing tabularx without any X type column is not correct. May 19, 2019 at 12:01

If you replace table by table* your table will span both of the two columns. In the following MWE, I have also used the mhchem and the siunitx package for chemical formulae and units. Lastly, I have removed all the parboxes and used tabularx's X type columns in order to get automatic linebreaks in the cells. I have also removed the justification in the cells as this causes quite large horizontal white spaces.

\documentclass[12pt,a4paper,twoside]{scrartcl}

\usepackage[utf8]{inputenc}
\usepackage[left={2.5cm},right={2.5cm},top={2cm},bottom={2.5cm}]{geometry}

\usepackage{tabularx,booktabs}
\usepackage[version=4]{mhchem}
\usepackage{siunitx}
\usepackage{ragged2e}

\DeclareSIUnit\Molar{M}
\DeclareSIUnit\Normal{N}

\usepackage{lipsum}
\begin{document}
\twocolumn

\begin{table*}[htbp]
\centering
\begin{tabularx}{\textwidth}{>{\RaggedRight}p{1.75cm}*{4}{>{\RaggedRight}X}}

\toprule
\textbf{step} & \textbf{extractant} & \textbf{target phase} & \bfseries possible \newline mechanism & \textbf{reference} \\
\midrule
\ce{PO4} & \SI{1}{\Molar} \ce{NaH2PO4}, pH\,5, \SI{16}{\hour}, \SI{24}{\hour} + water wash & adsorbed As & anion exchange of \ce{PO4} for \ce{AsO4} and \ce{AsO3} & \cite{welchFactorsControllingShallow1998}; modified by \cite{keonValidationArsenicSequential2001}\\
HCl & \SI{1}{\Normal} HCl, 2$\cdot$\SI{1}{\hour} + water wash & As coprecipitated with Mn oxides and amorphous Fe oxides & proton dissolution; Fe-Cl complexation & \cite{huerta-diazPyritizationTraceMetals1992}; \cite{cornwellCharacterizationIronSulfide1987}; \cite{wallmannNewProcedureDetermining1993}; \cite{canfieldSulfateReductionDiagenesis1988} \\
Microwave digestion & xxx & xxx & xxx & \cite{various} \\
\bottomrule
\end{tabularx}

\caption[Single Extraction: Steps]{Single Extraction Procedure; table from \cite{keonValidationArsenicSequential2001} (modified)}
\label{extraction_steps}
\end{table*}
\lipsum
\end{document}


Finally, a small comment about the contents of the table: There seem to be some missing charges especially regarding PO4, AsO4,...

• Thanks a lot, that's exactly what I wanted and it's even less complicated than what I've tried. As for the charges, that's how it was published in the original, I guess some simplification is ok here, since it isn't an equation but rather a short explanation of possible mechanisms May 19, 2019 at 14:03