## 3 Results and Discussion  
### DOC and Ni Concentrations and Mass Fractions in DOM and AEOM  
DOC concentration represents the abundances of dissolved organic matter [3; 36]. The mass fractions of Ni and DOC in size-fractioned DOM and AEOM were important to understand the Ni and DOC distribution between DOM and POM. Table 1 lists DOC and Ni concentrations of bulk and fractioned DOM and AEOM. In two other studies, concentrations of DOC in bulk DOM were similar to river water, which ranged from 1.3 to 20 mg/L with an average of 6 mg/L for DOC [12] but lower than sediment pore water, which ranged from 15-28 mg/L [37]. In our research, bulk alkaline extracted organic carbon (AEOC) averaged 14.78 g-C/kg and total organic carbon was 73.1 g-C/kg based on sediment mass. The AEOC/TOC ratio was 20.2%. The AEOC content in present study was higher than water extracted sediment organic carbon, which averaged 226 and 828 mg-C/kg [38] and alkaline extracted organic carbon from sediment, which averaged 5.94 \(\pm\) 0.56-6.40 \(\pm\) 0.96 g-C/kg [39]. The AEOC/TOC ratios in this study were higher than the WEOC/TOC ratios extracted from soil reported by [PERSON] et al. [22], and [PERSON] et al. [38] and comparable to the AEOC/TOC ratio, which averaged 23.7% from sediment [39]. The high AEOC concentration in this study suggested the sediment contained highly extractable organic matter, which could be discharged from food wastewater. The concentration of Ni in DOM was higher than river water and sediment pore water in other studies, which ranged from 1.05 to 7.52 ug/L [14; 23; 37; 40]. In present study, the average Ni concentration of bulk AEOM was 1.70 mg/kg and the total Ni concentration averaged 33.0 mg/kg, based on the sediment mass. The AEOM-Ni accounted for 5.2% of the total Ni. The AEOM-Ni concentration in the present study was higher than the water and NaOH-extracted Ni concentrations 0.21-1.21 mg/kg from soil [19; 22]. In addition, the ratio of AEOM-Ni/total Ni was higher than the ratio \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**DOM**} & \multicolumn{2}{c}{**AEOM**} \\ \cline{2-5}  & **DOC (mg/L)** & **Ni (ug/L)** & **DOC (mg/L)** & **Ni (ug/L)** \\ \hline Bulk & 8.3 \(\pm\) 0.4 a  & 5.56 \(\pm\) 2.16 & 739 \(\pm\) 33 & 85 \(\pm\) 7 \\ MW-A & 24.8 \(\pm\) 6.9 & 4.49 \(\pm\) 3.77 & 5040 \(\pm\) 231 & 520 \(\pm\) 82 \\ MW-B & 8.4 \(\pm\) 0.8 & 7.56 \(\pm\) 1.75 & 966 \(\pm\) 3 & 137 \(\pm\) 12 \\ MW-C & 9.6 \(\pm\) 1.6 & 6.23 \(\pm\) 1.49 & 834 \(\pm\) 141 & 95 \(\pm\) 19 \\ MW-D & 9.3 \(\pm\) 2.4 & 3.46 \(\pm\) 3.44 & 247 \(\pm\) 16 & 17 \(\pm\) 3 \\ MW-E & 6.3 \(\pm\) 1.0 & 6.71 \(\pm\) 3.96 & 145 \(\pm\) 9 & 6.0 \(\pm\) 0.0 \\ \hline \end{tabular}
\end{table} 
Table 1: The measured DOC and Ni concentrations for size-fractioned dissolved organic matter (DOM) and alkaline extracted sediment POM solution (AEOM) *. 1.07%, as reported by [PERSON] et al. [22]. The ratio of extracted organic carbon was higher than the Ni ratio, which suggested the organic matter was more readily extracted by alkaline solution than the Ni. The average mass balances ranged from 105 to 107% for OC and 91 to 111% for Ni in DOM/AEOM, which were within reasonable ranges (100 \(\pm\) 25%, [PERSON] et al. [37]). For example, [PERSON] et al. [39] separated the sediment alkaline-extracted organic matter into three size-fractionated AEOM, 10 kDa-0.45 \(\upmu\)m, 1-10 kDa, and \(<\)1 kDa, where the organic carbon mass balances were 92 \(\pm\) 15% to 95 \(\pm\) 8%. [PERSON] et al. [22] separated 24 soil alkaline extracted solutions into 1 kDa-0.45 \(\upmu\)m and \(<\)1 kDa fractions and the organic carbon mass balances were averaged 90 \(\pm\) 26% and mass balances for Ni were averaged 83 \(\pm\) 14%. [PERSON] et al. [37] separated five-sediment pore water DOM into 5 kDa-0.45 \(\upmu\)m and \(<\)5 kDa fractions and the Ni mass balances were 100-280%. In Table 1, the DOC and Ni concentrations of size-fractionated DOM/AEOM were the measured concentrations.